2026 Social Media Platform Format Specs: The Complete Field Guide

Compiled from public platform developer documentation and independent industry research (Buffer, Hootsuite, Later, Socialinsider, HubSpot). Last updated April 2026.

Why This Guide Exists

Every content creator, marketer, and founder has run into the same problem: you're about to ship a post, and you need the exact image dimensions for an Instagram carousel, the current character ceiling on X, or the precise aspect ratio TikTok's algorithm actually rewards. You open Google. You find a Hootsuite article from 2022, a Sprout Social post from 2023, and a Reddit thread arguing about whether LinkedIn still penalizes outbound links.

Nobody has compiled this in one place for 2026. Not because it's hard - but because platforms change specs quietly, studies get buried behind email gates, and most "ultimate guides" stop updating the week they're published.

This guide is a reference, not a listicle. Every number is cited to either the platform's own developer docs or a reputable industry study. When a spec is contested, we give you the range and the source - not a false single answer. Bookmark it. Come back when you need the number.

1. Twitter / X

Character limits: Post 280 chars (free) / 25,000 (Premium). Bio 160. Display name 50. Alt text 1,000. DM 10,000.

Image specs: In-feed 1600x900 px landscape or 1080x1080 square. Header 1500x500. Profile 400x400 (circle). Card image 1200x628 min (2:1). Max 5 MB JPG/PNG.

Video specs: Aspect 1:1, 16:9, or 9:16. Max 2:20 (free), 3 hours (Premium). Max 512 MB web / 1 GB Premium. H.264 + AAC MP4.

Optimal posting times: Buffer's 2024 analysis of 1M+ tweets found weekday mornings (9-11am local) dominate engagement, with a secondary spike at 1-3pm. Hootsuite 2024 independently confirmed Wednesday and Friday 9am as peak. Weekends underperform weekdays by ~20%.

Algorithm signals: Per X's open-source algorithm repo (github.com/twitter/the-algorithm), replies are weighted ~27x a favorite, retweets ~20x, and "profile clicks following a tweet" is one of the highest-value signals. Posts with outbound links are down-ranked; posts with images or video get a visibility boost.

Link policy: External links demoted in algorithmic ranking per Musk's confirmed 2023 policy change. Common workaround: post the thread natively, put the link in a reply.

Hashtags: 1-2 is the documented sweet spot (Buffer 2024). Placement: end of post or mid-sentence. CamelCase improves screen-reader accessibility.

Non-obvious insight: X treats a bookmarked post as near-equivalent to a retweet in ranking weight - a signal most creators ignore because bookmarks are private. This is why "save-worthy" threads dramatically outperform hot takes over a 48-hour window.

2. LinkedIn

Character limits: Post 3,000. Headline 220. About 2,600. Article 110,000. Comment 1,250. Company tagline 120.

Image specs: Feed 1200x627 landscape, 1200x1200 square, or 1080x1350 portrait (highest engagement per Socialinsider 2024). Personal cover 1584x396. Company cover 1128x191. Profile 400x400. Max 5 MB.

Video specs: 1:1, 16:9, or 9:16 (for LinkedIn Video short-form). Max 10 min native feed, 30 min Live. Max 5 GB. H.264 + AAC MP4.

Optimal posting times: Hootsuite 2024 LinkedIn study - Tuesday-Thursday 10am-noon local time is the clear peak. Buffer 2024 corroborated with Tuesday 10am as single highest slot. Mondays underperform (inbox clearing). Fridays drop after 2pm.

Algorithm signals: LinkedIn Engineering's 2023 "Improving Feed Ranking" post states the platform optimizes for comments (heaviest weight), reshares with commentary, and dwell time. A like is worth a fraction of a comment.

Link policy: LinkedIn demonstrably down-ranks posts with outbound links in the body. Best practice confirmed by Richard van der Blom's 2024 algorithm research: post without a link, add the link in the first comment.

Hashtags: 3-5 per LinkedIn's own documentation. Place at end. CamelCase preferred for accessibility.

Non-obvious insight: LinkedIn rewards dwell time more than any other major platform. A 1,500-character post that keeps a reader scrolling for 8 seconds beats a 400-character post that earns a quick like. This is why "three-line hook + see more cliff + payoff" structures work - they force expansion clicks, a measurable dwell-time signal.

3. Instagram

Character limits: Caption 2,200. Bio 150. Comment 2,200. Username 30. Alt text 100 (manual override).

Image specs: Feed square 1080x1080. Feed portrait 1080x1350 (4:5, maximum vertical real estate). Landscape 1080x566 (1.91:1). Story/Reel 1080x1920 (9:16). Profile 320x320 (displays 110x110). Max 30 MB.

Video specs: Reels 9:16, up to 90 seconds standard or 3 minutes extended (2024 rollout). Feed 1:1 or 4:5, up to 60 min. Max 4 GB. H.264 + AAC MP4/MOV.

Optimal posting times: Later's 2024 analysis of 11M Instagram posts: Monday-Friday 9am-11am local is peak. Hootsuite 2024: Tuesday 11am is single highest slot globally. Sunday consistently underperforms.

Algorithm signals: Per Adam Mosseri's 2023 "How Instagram Works" post, the feed algorithm weighs time spent on post, likes, comments, saves, shares, and profile visits following a view. Saves and shares are the two most valuable signals for Reels distribution.

Link policy: No clickable links in captions. Links work only in bio, Stories (via link sticker, available to all accounts as of 2022), and DMs. Standard workaround: "link in bio" + Linktree equivalent.

Hashtags: Instagram's 2023 @creators guidance - 3-5 hashtags perform best. The "use all 30" strategy is dead. Socialinsider 2024: 3-5 hashtags outperform 11+ by ~1.3x reach. Place in caption or first comment (functionally equivalent).

Non-obvious insight: Per Socialinsider's 2024 benchmark (77M posts analyzed), carousels outperform single-image posts by approximately 1.4x in engagement rate. Carousels with 7+ slides outperform shorter carousels because Instagram re-serves the post to users who didn't swipe past slide 1 - effectively giving each post two shots at the algorithm.

4. Facebook

Character limits: Post 63,206 chars (engagement drops sharply past 80 chars per Buffer). Page bio 255. Page name 75. Comment 8,000.

Image specs: Feed 1200x630 recommended (min 600x315). Cover 851x315 (displays 820x312 desktop, 640x360 mobile). Profile 170x170 displayed (upload 320x320+). Event cover 1920x1080. Max 30 MB.

Video specs: Feed 1:1 (highest mobile engagement), 16:9, or 9:16 Reels. Max 240 min. Max 10 GB. H.264 + AAC.

Optimal posting times: Hootsuite 2024 - Monday-Friday 8am-noon local. Buffer 2024: Tuesday, Wednesday, Friday 9am peak. Facebook's audience skews older than Instagram's, which is why morning windows outperform evening.

Algorithm signals: Meta's documentation emphasizes "meaningful social interactions" (MSI) - comments and replies between users weigh more than likes. Video watch time and native video uploads are rewarded over external video links (YouTube links in particular are down-ranked).

Link policy: External links demoted relative to native content but less aggressively than LinkedIn or X. Link preview cards (Open Graph) render cleanly with correct OG tags. Meta's 2016+ clickbait classifier still operates.

Hashtags: Almost no organic reach benefit on Facebook (Buffer's 2023 study). 1-2 is the functional maximum.

Non-obvious insight: Facebook's algorithm heavily rewards posts that generate reply chains (comment -> reply -> reply-to-reply). A single 10-reply conversation on one comment outperforms ten independent one-off comments. This is why posts asking genuinely open-ended questions consistently beat statement posts in reach.

5. TikTok

Character limits: Caption 4,000 chars (expanded from 2,200 in 2023). Bio 80. Username 24. Comment 150.

Image specs: Photo post 1080x1920 (9:16). Profile 200x200 (circle). Max 20 MB per image.

Video specs: 9:16 only - anything else gets letterboxed and de-prioritized. Max 10 min standard, 60 min in test/select regions. Max 287.6 MB iOS / 72 MB Android via app / 4 GB via web. H.264 or H.265; MP4 or MOV.

Optimal posting times: Later's 2024 TikTok study (1M videos) - Tuesday 9am, Thursday noon, Friday 5am local time as top three. Influencer Marketing Hub 2024 corroborated Tuesday/Thursday weighting. TikTok's Creator Portal recommends checking your own Analytics > Followers for audience activity.

Algorithm signals: Per TikTok Newsroom's "How TikTok recommends videos #ForYou" (2020, still canonical): watch time, completion rate, re-watches, shares, comments, follows from a video. Watch time and completion rate dwarf every other signal.

Link policy: No clickable links in video captions. Bio link available to accounts with 1,000+ followers. External links in DMs. TikTok Shop links are the major native exception.

Hashtags: 3-5 - mix one broad (#marketing) with 2-3 niche (#b2bsaas, #contentops). End of caption.

Non-obvious insight: TikTok has a documented first-90-seconds retention cliff. Videos where >50% of viewers drop off before the 90-second mark (or 3-second mark for short videos) stop getting served to new audiences almost immediately. Hootsuite 2024: videos with a visual change in the first 2 seconds see ~1.6x the completion rate of static-start videos.

6. YouTube

Character limits: Video title 100 chars (first 60 visible in most surfaces). Description 5,000. Tags aggregate 500. Community post 1,500. Comment 10,000. Channel name 100.

Image specs: Thumbnail 1280x720 (16:9), max 2 MB, JPG/PNG/GIF/BMP. Channel banner 2560x1440 (safe area 1546x423). Profile 800x800 (displays 98x98). Shorts cover 1080x1920.

Video specs: Long-form 16:9 (1920x1080 1080p, 3840x2160 4K). Shorts 9:16 up to 60s (3-min Shorts rolling out mid-2024). Max 256 GB or 12 hours. H.264 recommended, H.265/VP9/AV1 accepted. AAC-LC audio.

Optimal posting times: HubSpot 2024 and Tubics data agree on weekday 2pm-4pm local for long-form (gives the video time to accumulate early watch signals before the 7-10pm prime viewing window). Shorts: morning and midday (TikTok-like).

Algorithm signals: Per YouTube Creator Academy - click-through rate on thumbnail, average view duration, average percentage viewed, and session time (did the viewer keep watching YouTube after your video?). Session time is the single most under-discussed metric - it's why recommendations favor videos that lead viewers deeper into the platform.

Link policy: Links in description are clickable but open same tab - low friction. Pinned comment links convert noticeably better than description links per most creator benchmarks. YouTube demotes videos with spammy description link farms.

Hashtags: Up to 15 in description; only first 3 render above the title. Overuse (>15) triggers tag-spam penalty per YouTube's documented policy.

Non-obvious insight: YouTube weighs the first 24-48 hours of watch signals disproportionately - the "velocity window." If CTR and retention clear channel-average thresholds in those first two days, YouTube promotes the video to progressively wider audiences. Miss the threshold and the video is effectively frozen out. This is why creators obsessively A/B test thumbnails in the first 48 hours (TubeBuddy, YouTube's native thumbnail test feature exist exactly for this).

7. Threads

Character limits: Post 500 chars. Bio 150. Username inherited from Instagram.

Image specs: Feed 1080x1350 (4:5, Instagram-inherited). Up to 10 images per post. Max 8 MB.

Video specs: 9:16 or 1:1 preferred. Max 5 min. Max ~1 GB. H.264 + AAC MP4.

Optimal posting times: Threads is new enough that canonical third-party studies are limited. Buffer's early 2024 Threads data suggests weekday mornings (8-10am) mirror Instagram patterns, tracking the shared identity graph. Meta has not published official audience-activity data.

Algorithm signals: Per Mosseri's public statements (2023-2024), Threads is optimized for replies and re-posts. Like Instagram, saves and shares weigh heavily. Threads explicitly does not boost news/politics content as of 2024 policy - a documented algorithmic de-prioritization.

Link policy: External links are clickable in post bodies without penalty - one of the few major platforms where this is true as of 2026. Deliberate differentiation from X.

Hashtags: One "topic tag" per post (Threads' native hashtag equivalent, 2024 rollout). Additional text hashtags render but carry no algorithmic weight.

Non-obvious insight: Threads' algorithm, unlike X's, actively surfaces replies from accounts you don't follow inside threads you're already reading. This means writing a thoughtful reply on a large creator's post is a legitimate, zero-cost growth tactic on Threads - something that stopped working on X years ago.

8. Email

Length targets: Subject 30-50 chars (open rate peaks at 41 per Mailchimp 2024). Preview 85-100 chars (Litmus 2024). Sender name under 25 chars for mobile. Body 50-125 words for max reply rate (Boomerang/HubSpot 2024). CTA button 2-5 words.

Image specs: Email width 600 px maximum (universally supported across Gmail, Outlook, Apple Mail). Hero 600x300. Retina-friendly: upload at 2x (1200 px) and display at 600 px. Max email size: 102 KB in Gmail before clipping - the single most important technical spec most email marketers don't know.

Video: Embedded video generally doesn't render inside Outlook, Gmail app, or most clients. Standard practice: static thumbnail with play-button overlay linking to hosted video.

Optimal sending times: HubSpot's 2024 analysis of 4B sends - Tuesday 10-11am local is the global peak for B2B. Mailchimp 2024: Tuesday/Thursday 10am for consumer. Avoid Monday morning inbox-flood and Friday afternoon drop-off.

Deliverability signals: Gmail's documented inbox placement factors - sender reputation (domain + IP), engagement rate (opens, replies, forwards), spam complaint rate (keep under 0.1% per Google Postmaster), authentication (SPF/DKIM/DMARC all passing), list hygiene (bounce rate under 2%).

Link policy: Links are fundamental. Best practice: 1 primary CTA, max 3 total links. Excessive linking (10+) trips spam classifiers.

Non-obvious insight: Gmail clips any email over 102 KB of HTML - not total email size, but the HTML body itself. Clipped emails hide your unsubscribe link (a CAN-SPAM risk) and your footer tracking pixel (destroying open-rate data). Most template builders produce 80-95 KB HTML by default, which is why adding a few heavy images or inline CSS can silently push you over.

9. Blog / Long-form

Length targets: SEO cornerstone 1,500-2,500 words (Backlinko 2024's 11.8M-result analysis - top-10 Google results average 1,447 words). Listicle 1,800-2,500. How-to 1,700-2,100. News 300-700.

Image specs: Featured/OG image 1200x630 (1.91:1) - simultaneously the Facebook/LinkedIn/Twitter card standard. Inline max-width 1200 px (display at container width, retina 2x). Format: WebP with JPG fallback. Alt text on every non-decorative image (WCAG 2.1 AA compliance + documented Google ranking factor).

Structural SEO: Exactly one H1 per page. H2/H3 hierarchical - don't skip levels. 3-5 internal links per 1,000 words. 1-3 outbound links to authoritative sources per 1,000 words (correlated with higher rankings per Moz).

Non-obvious insight: Google's Helpful Content system (2022+, reinforced through 2024 core updates) penalizes sites that publish thin content even when individual posts are high-quality. A single great article on a site with 200 mediocre posts underperforms the same article on a site with 50 consistently-good posts. Pruning low-performers is an actual SEO tactic, not just a cleanup exercise.

10. SEO / Meta Tags

Length specs: <title> 50-60 chars (~600 px SERP render width). Meta description 150-160 chars desktop / 120 mobile. OG title 60 chars. OG description 2-4 sentences (~200 chars). Twitter card title 70 chars. URL slug 3-5 words, under 75 chars total URL (Backlinko 2024: short URLs correlate with higher rankings).

Image specs: Open Graph image 1200x630 (1.91:1), under 5 MB, JPG/PNG. Twitter summary_large_image 1200x628, max 5 MB. Favicon 32x32 (serve a 512x512 source for high-DPI). Apple touch icon 180x180.

Schema.org priorities: Highest-ROI types per Google's documented rich-result support - Article/BlogPosting (rich article cards), FAQPage (FAQ accordion), HowTo (step-by-step cards, restricted post-2023 but still viable for some verticals), Product/Review (stars + price), Organization + Person (knowledge panel eligibility).

Non-obvious insight: Google ignores your meta description roughly 63% of the time and writes its own from page content (Ahrefs' 2024 study of 85,000 queries). Your meta description is less of a ranking tool than a click-through optimizer for the 37% of queries where Google actually honors it. Spend the effort on title tags, where Google honors authorship ~85% of the time.

Cross-Platform Insights

1. Posts that work on X rarely work on LinkedIn - here's why

X rewards reaction speed (replies within minutes). LinkedIn rewards dwell time (seconds spent reading). A punchy one-liner thrives on X and dies on LinkedIn because it generates no dwell. A three-paragraph reflection thrives on LinkedIn and dies on X because nobody scrolls to read it. These are opposing optimization targets, not a single "social voice."

2. 1080x1350 (4:5 portrait) is the most universally useful image dimension in 2026

It's Instagram's maximum feed height, LinkedIn's highest-engagement portrait format (Socialinsider 2024), and crops cleanly to 1:1 for X and Facebook. If you produce one image and deploy it everywhere, 1080x1350 is the pragmatic default.

3. Short-form video specs have converged

TikTok, Reels, Shorts, LinkedIn Video, and Pinterest Idea Pins all accept 1080x1920 9:16 H.264 MP4. Produce once, deploy five times. The only platform-specific change is caption/hashtag norms - not the video file.

4. The link-friction rank is: Threads < Facebook < YouTube < X < LinkedIn ≈ Instagram

Threads rewards links. Facebook tolerates them. YouTube is neutral. X demotes them. LinkedIn and Instagram effectively block them from the feed. Plan your CTA placement accordingly.

5. Every major platform rewards replies/comments over likes

X algorithm weights: reply 27x > like. LinkedIn: comment > reshare > like. Facebook: MSI favors comment threads. TikTok: comments are a top-5 signal. Instagram: comments > likes in Reels distribution. If your content strategy optimizes for likes, you're optimizing for the wrong metric on every platform.

6. Hashtag counts have collapsed across platforms

The 2020 era of "max out your hashtags" is over. Current consensus across Buffer, Later, Socialinsider, and the platforms' own docs: 3-5 hashtags on Instagram, TikTok, and LinkedIn. 1-2 on X and Facebook. More than that is neutral-to-negative everywhere.

One Tool That Respects Every Spec in This Guide

Knowing these specs is half the battle. Hitting them on every post is the other half.

BriskTool's Content Machine generates platform-specific drafts that respect every constraint in this guide: character counts per platform, image dimensions per placement, hashtag counts per algorithm, link placement per policy. One brief in, 10 platform-ready assets out, each formatted to the 2026 spec it needs to hit.

Sources

Platform developer / official documentation

• X (Twitter) Developer Platform - developer.x.com/en/docs
• X Algorithm open-source repo - github.com/twitter/the-algorithm
• LinkedIn Developer - learn.microsoft.com/en-us/linkedin/
• LinkedIn Engineering Blog (Feed Ranking)
• Instagram "How Instagram Works" (Mosseri, 2023)
• Meta for Developers (Facebook Graph API)
• TikTok Newsroom "How TikTok recommends videos"
• TikTok Creator Portal
• YouTube Creator Academy + YouTube Help upload specs
• Threads Help Center
• Google Search Central + Google Postmaster Tools

Industry studies cited

• Buffer 2024 "Best Time to Post on Social Media" (1M+ posts)
• Hootsuite 2024 Social Media Benchmarks Report
• Later 2024 Instagram and TikTok posting-time studies
• Socialinsider 2024 Instagram Benchmark Report (77M posts)
• HubSpot 2024 State of Marketing Report
• Richard van der Blom's annual LinkedIn Algorithm Research, 2024 edition
• Backlinko 2024 Google Ranking Factors study (11.8M SERP results)
• Ahrefs 2024 meta description study (85,000 queries)
• Mailchimp 2024 Email Marketing Benchmarks
• Litmus 2024 State of Email Report

This guide will be updated quarterly as platforms change specs. Last revised April 2026.