What this guide does: Explains what Instagram and TikTok actually reward, and turns each mechanic into a rule you can build a post against. Read it before deciding a format, a slide count or a caption length, and when a post that looked good got no reach.
Whether a post is good and whether it travels are two different questions. This one is about the second. Every rule here exists because of a specific mechanic, and if the mechanic changes the rule goes with it - so each is written with its reason attached rather than as a commandment.
Verified against published sources 2026-08-04. Platform mechanics decay. Treat anything here as a claim about a moving system, re-check it quarterly, and prefer your own numbers the moment you have them.
The three named ranking signals are watch time, sends per reach, and likes per reach. Sends are the heaviest, reported at roughly three to five times the weight of a like. Sends per reach is specifically the signal that reaches people who do not follow you, which is the only reach a new account can grow on.
Feed, Reels, Stories and Explore rank separately and weight those signals differently. Feed leans on how close you already are to the viewer; Explore on engagement velocity and interest match. A post that does well with your existing followers is not automatically a post that travels.
Why carousels, specifically:
- Every swipe is engagement, and dwell time accumulates across the slides. Carousels are reported at two to three times the reach of a single image for the same content.
- The platform re-serves a carousel to people who did not swipe, starting from the second slide. This is the most actionable mechanic available: slide two gets an independent second chance to be someone’s first impression.
- Carousels out-save single images by a wide margin, and saves compound, because saved posts get resurfaced.
- Completion - how many people reach the last slide - is what pushes a post out of your followers and into Explore.
Hashtags do not drive distribution. The platform’s own position is that they categorise rather than distribute. Discovery comes from the words in your caption: captions, alt text, bios and on-screen text are indexed and served into in-app search. Keyword-rich captions have been measured at around 30% more reach than hashtag-heavy ones. Keep three to five hashtags as labels and spend the effort on the prose.
TikTok, photo mode
- Ranking is swipe-through rate, dwell time and reverse swipes, with completion rate as the primary signal.
- Distribution starts with a small test batch of roughly 200-500 viewers, mostly followers and people who engage with adjacent content. What that batch does decides everything afterwards. This is why the first hour matters, and why a weak second slide is fatal rather than merely costly.
- Saves are weighted and photo posts save well. A carousel with fewer views but a high save-and-comment share is doing better than a higher-view post that bounces on slide one.
- Interest-based distribution means an outlier is possible from your first post. TikTok is spikier; Instagram grinds.
What follows for how you build a post
| Rule | Because |
|---|---|
| Slide 2 is a second hook. It delivers on slide 1 and opens a new thread, and it has to work cold | the platform re-serves from slide 2; TikTok’s test batch dies there |
| Seven slides for a narrative format (hook, five body, closer) | seven to ten is the reported dwell-time sweet spot; under five reads as a short post, over ten causes mid-carousel fatigue |
| No slide may be skippable. If a body slide can be removed without breaking the post, you wrote a list, not an experience | completion is the ranking signal, and a list lets people stop anywhere |
| The peak lands in the last third | a middle peak makes the tail a letdown, and the tail is where completion is won |
| Design for the send, not the like. At least one slide should make someone think of a specific person | sends per reach is the non-follower signal, worth several likes |
| The send prompt lives in the caption, naming one kind of person, never “share if you relate” | it belongs where sends are earned, and it keeps the last slide screenshottable |
| At least two slides hand over something usable | recognition earns likes; recognition plus something doable earns saves, and saves compound |
| Captions carry the audience’s own search phrases, in prose | captions are indexed; hashtags are not distribution |
| Caption length 400-600 characters on Instagram, 150-300 on TikTok, first line an independent hook under 125 characters | that is where the “more” cut falls; past roughly 300 characters TikTok needs a tap and reach drops |
| Three to five hashtags, never the generic feed tag | labels, not reach |
| One canvas at 4:5, content clear of the top and bottom edges | survives the 3:4 grid crop, letterboxes acceptably on TikTok |
Cadence, and the first hour
Three to five posts a week, sustained. Three a week for twelve weeks beats seven a week for three. The failure mode is not low quality, it is stopping - and the documented version of it is 34 drafts and 3 published posts.
The first hour is the test batch. Being there to answer early comments is the cheapest intervention available on either platform; a comment answered in the first hour is worth more than the same reply a day later.
Post at a fixed time your audience is awake for, set once on the account’s posting schedule rather than decided per post.
Expect silence for two months. A new account with no face takes three to six months to reach a thousand followers on Instagram. Distribution is a power law: a few posts carry most of the reach. Judging before 90 days is judging noise.
When something works, fan it out
A post that breaks out is the beginning of the work, not the end.
- Make five to ten variants of the winner with the structure fixed and exactly one variable changed - the hook phrasing, the register, the opening image.
- One variable per variant, or the result teaches nothing.
- Log which variant won and which source line its hook came from. That tells you which well to keep digging.
- A losing variant is data. Kill it rather than nursing it.
What is worth not believing
Everything above is published guidance and platform statements, not our measurements. Before you treat any of it as settled for your audience, these are the clean one-variable tests: caption length judged on saves and sends per reach; a quiet text card against a photo behind text; a native 9:16 crop against a letterboxed 4:5; whether a screenshot of your app costs reach or buys installs.
Log the real numbers per post at seven days - views, completion, saves, sends, comments, profile taps. The moment you have your own numbers they outrank every source below.
Sources
- Instagram algorithm ranking signals (Buffer) - watch time, sends per reach, likes per reach, per-surface ranking
- The ranking signals that matter (Clixie) - sends weighted three to five times a like
- How carousels beat Reels for engagement (Storrito) - re-serving from slide 2, reach multiple
- Carousel best practices (Adpicto) - slide count, dwell time, saves
- Carousel algorithm (TryMyPost) - dwell time and completion
- Do hashtags still work (Kontentino) - hashtags do not drive reach
- Keywords versus hashtags (Dive Media) - caption indexing and the reach lift
- TikTok photo mode algorithm (ReelBase) - photo mode reach against video
- TikTok carousel algorithm (PostWaffle) - swipe-through, reverse swipes, test batch size, saves