Best Weight Gain Apps in 2026: The Ones That Actually Set a Surplus
Short answer
Most apps sold for weight gain are shake calculators, and most calorie trackers assume you want a deficit. Four apps state that they set a genuine surplus goal: SlimAI, MacroFactor, MyFitnessPal and Fitia. Cronometer, Lose It! and Cal AI mention gain without documenting a surplus goal, and should be treated as trackers you point upward yourself.
Gaining weight deliberately is the same arithmetic as losing it, run in reverse, and almost none of the software is built for that direction. Search the app stores for weight gain and most of what returns is a shake calculator or a supplement brand. Search for a calorie tracker and you get products whose defaults, copy and progress charts all quietly assume you want the number to go down.
So the useful question is not which app has the nicest interface. It is whether the app will compute a surplus for you, stop you from choosing a reckless one, and register a heavier weigh-in as the thing you asked for rather than a setback.
This page sorts the category on that question, using each vendor's own App Store listing or site as the source, read 4 September 2026. SlimAI makes one of the apps here and is listed first, which is a claim this page then has to earn — the surplus figures, the caps and the underweight floor below are specific because they are checkable against the product, not because they read well.
Does the app actually set a surplus?
The distinction in that table matters more than it looks. Any calorie tracker can be used to gain weight — you type a bigger number and eat to it. What separates the top four is that the app derives the number, which means it also recalculates when your weight moves, and it does not treat the recalculation as a failure.
| App | Surplus goal | What the source says |
|---|---|---|
| SlimAI | Yes, with caps | Gain Weight is one of three goals at setup and can be changed later. The surplus is +250, +500 or +750 kcal by intensity, capped at 1,000 kcal or 50% of daily energy expenditure, whichever is lower. |
| MacroFactor | Yes | App Store description: “Set a goal to lose, maintain, or gain weight.” No free tier — a 7-day trial, then subscription only. |
| MyFitnessPal | Yes | App Store description: “Customize calorie, nutrition & fitness goals – weight loss, gain, maintenance, or performance.” |
| Fitia | Yes | App Store description: “Personalized meal plans for weight loss or muscle gain”, with a section headed “Why Fitia works for weight loss or muscle gain”. |
| Cronometer | Mentioned, not documented | “Perfect for weight loss, muscle gain, and improving overall health” appears in the listing, but no source describes a gain goal in the app’s goal setup. |
| Cal AI | Mentioned, not documented | “Muscle Building” appears in the Google Play listing only. No source states that a gain goal or a surplus target can be selected. |
| Lose It! | Not documented | Across the App Store listing and the vendor site the word “gain” appears once, in a bullet about device syncing — never in goal setup. |
| Protein Pal | Protein only | Its “Build Muscle” option sets a protein target, not a calorie surplus. Useful alongside a tracker, not instead of one. |
Every cell traces to that vendor's own App Store listing or website, read 4 September 2026. “Mentioned, not documented” means the vendor names gain in marketing copy but no source describes the goal setting — treat those as trackers you can point upward manually rather than apps built for it. Tiers and copy change; check the listing before subscribing.
The safety question nobody in this SERP asks
A deficit has a natural floor: eat too little and you feel it immediately. A surplus has no equivalent brake. Type +1,500 into a tracker that will accept it and you will gain, mostly as fat, and the app will congratulate you the whole way.
That makes the ceiling the most important number in a gain app, and it is the one no listicle on this SERP publishes. SlimAI caps the surplus at 1,000 kcal or 50% of your total daily energy expenditure, whichever is lower, so the cap tightens for smaller people rather than applying one number to everyone. Where a user has recorded heart disease, the surplus is clamped to 500.
The opposite guard matters too. A user with a BMI under 18.5 and no recorded medical condition is given a floor of +500 rather than the gentler option, because a nominal surplus for someone underweight is functionally maintenance. If you are gaining from underweight, that floor is the feature to look for in any app you pick.
None of this is medical advice and none of these apps are a clinician. If you are underweight for a reason you have not identified, or gaining on a doctor’s instruction, the app is for the logging, not the plan.
What each app is genuinely best at
SlimAI — a surplus with brakes, and logging fast enough to hit it
The problem with gaining is volume: you have to eat more than feels natural, every day, and log it. SlimAI computes the surplus (+250, +500 or +750), caps it, adjusts it as your weight moves, drops your step goal by 2,000 on a gain goal so activity is not quietly eating the surplus, and counts a heavier weigh-in as progress. One AI photo scan a day is free, so the logging is a photo rather than six database searches. Custom gram-level macro targets are SlimAI Pro.
MacroFactor — the most rigorous surplus, at full price
Its expenditure model estimates what you are actually burning from your logged intake and weight trend, then adjusts the target weekly. For gaining, that is the most quantitatively honest approach available, because it catches the metabolic adaptation that makes a fixed surplus stop working. There is no free tier at all — a 7-day trial, then subscription only.
MyFitnessPal — the largest database, if you eat packaged food
Gain is a first-class goal and the food database is the biggest in the category, which matters when you are eating high volume and reaching for convenience food to hit the number. The cost is that barcode scanning, photo logging and voice logging are all Premium, so the free experience is typing.
Fitia — meal plans built for a surplus
Its own copy leads on plans for muscle gain, which suits people who would rather be told what to eat than assemble a surplus themselves. Note before choosing it: its comparison table marks photo, voice, text and barcode logging as Premium, and the free food database as “Limited”.
Protein Pal — a companion, not a tracker
Protein-first by design, with free manual logging on both platforms. Its muscle-building goal sets a protein target and no calorie target, so it cannot run a surplus on its own. Reasonable alongside a calorie tracker if protein is the number you keep missing.
Choosing in one minute
You are underweight and gaining on purpose
Pick an app with a documented surplus and a floor rather than one you point upward yourself. Check what the app does at a BMI under 18.5 before you trust its number.
You are lean bulking and already know your macros
MacroFactor for the adaptive expenditure model, if you will pay for it. Otherwise a tracker with gram-level macro targets — that is a paid feature in SlimAI and MyFitnessPal alike, and free in Cronometer.
You keep failing to eat enough because logging is tedious
The binding constraint is input speed, not the target. Look for photo or voice logging on the free tier — that is SlimAI’s one scan a day, or Cronometer’s Gold if you will pay.
You want a plan rather than a target
Fitia, with its Premium boundary understood up front.
Sources
App Store listings and vendor sites for MacroFactor (macrofactorapp.com), MyFitnessPal (myfitnesspal.com/premium and its Free/Premium/Premium+ support table), Fitia (fitia.app/premium), Cronometer (cronometer.com/gold), Lose It! (loseit.com/how-it-works), Cal AI (calai.app and its Google Play listing) and Protein Pal (proteinpalapp.com).
All read 4 September 2026. SlimAI's surplus tiers, caps, underweight floor and adaptive multipliers are taken from its own product specification and shipped code rather than from marketing copy, which is why they are quoted as figures.
What the research says
Independent findings, not ours
Two things are worth knowing before picking any app on this page. Neither study tested SlimAI; both describe the category.
Across 90 adults tracking food in an app for eight weeks, the people who completed the study logged on 70.2% of days and lost 2.19% of body weight, against 1.54% for the sample as a whole, which logged on about half of days. Frequency of logging explained 6.8% of the variance in weight change.
Thirty dietary recalls were entered into MyFitnessPal, Fitbit, Lose It!, MyPlate and Lifesum and compared with the NDSR research database. Energy and macronutrient correlations ran 0.73–0.96, but individual nutrients were underestimated by 7–41% depending on the app — MyFitnessPal for protein, total fat, sodium and cholesterol; Lose It! across six nutrients.
FAQ
Still Wondering About SlimAI
What is the best app for gaining weight?
What is the best app for gaining weight?Of the apps checked, four state that they set a genuine gain goal rather than only mentioning gain in marketing copy: SlimAI, MacroFactor, MyFitnessPal and Fitia. Pick between them on what actually stops you — SlimAI if the daily logging is what defeats you and you want a capped surplus computed for you, MacroFactor if you want the target re-derived weekly from your own data and will pay for it, MyFitnessPal for database size, Fitia if you want a meal plan rather than a target.
How big should a calorie surplus be?
How big should a calorie surplus be?Smaller than most people assume, because the surplus above your needs is what determines how much of the gain is fat rather than lean mass. SlimAI offers +250, +500 or +750 kcal a day depending on how fast you want to move, and refuses to go above 1,000 kcal or 50% of your total daily energy expenditure, whichever is lower. Treat any app that lets you set an unlimited surplus as a spreadsheet rather than a coach, and talk to a clinician if you are gaining for a medical reason.
Do calorie trackers work for weight gain, or only weight loss?
Do calorie trackers work for weight gain, or only weight loss?The arithmetic works in both directions, but the software often does not. Many trackers assume a deficit in their defaults, their copy and their progress charts, so a heavier weigh-in reads as a failure even when it is exactly what you asked for. Check three things before committing: that gain is a selectable goal, that the target recalculates as your weight changes, and that a rising weight is shown as progress.
Are weight gainer apps the same as calorie trackers?
Are weight gainer apps the same as calorie trackers?No, and the app stores blur them. Most products sold as “weight gainer” apps are shake or supplement calculators — they tell you what to drink, not what you have eaten. A calorie tracker with a gain goal does the opposite. If you are gaining seriously you want the tracker; a gainer shake is one entry in it.
Does SlimAI support a weight gain goal?
Does SlimAI support a weight gain goal?Yes. Gain Weight is one of three goals at setup and can be switched later from your profile. It produces a real surplus of +250, +500 or +750 kcal a day depending on the intensity you pick, capped at 1,000 kcal or 50% of your daily energy expenditure; the cap tightens to 500 where heart disease is recorded, and a user with a BMI under 18.5 and no recorded condition is given a floor of +500. Your step goal drops by 2,000 on a gain goal so activity does not quietly cancel the surplus, and a weigh-in that goes up is recorded as progress.
Which weight gain apps are free?
Which weight gain apps are free?All of the ones here are free to download, and that is not the same as usable free. MacroFactor has no free tier at all — a 7-day trial, then subscription. MyFitnessPal is free to log by typing, with barcode, photo and voice logging in Premium. Fitia marks photo, voice, text and barcode logging as Premium and its free food database as “Limited”. SlimAI includes one AI photo scan a day free with no time limit, along with the fasting timer, water, workouts, steps and adaptive calorie goals; custom gram-level macro targets are Pro.
How fast should I expect to gain?
How fast should I expect to gain?A +500 kcal surplus is roughly half a kilogram, or about a pound, a week in theory — and reality runs slower and noisier than the arithmetic, because energy expenditure rises as you eat more and scale weight swings with water, food volume and glycogen. Judge a gain phase on a multi-week trend rather than any single weigh-in, and if the trend is flat for a fortnight of consistent logging, the surplus is the thing to revisit.
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