How our calculators work

Your maintenance calories come from the National Academies' 2023 energy equations, your resting burn from Mifflin-St Jeor, and your weight projections from the NIH dynamic weight-change model — with the old 3,500-calorie rule shown only for comparison. Every number on the site is produced by one calculation engine and checked automatically against its sources, so what you read on a page can't drift from the math behind it.

Maintenance calories

Your maintenance — the calories you burn in a day, also called total daily energy expenditure — comes from the 2023 National Academies energy equations (Dietary Reference Intakes for Energy, Table S-1). Those equations were fitted to doubly labeled water measurements, the benchmark method for measuring how much energy people actually burn as they go about normal life, rather than to resting-rate estimates scaled by a guess.

The equation takes your sex, age, height and weight, plus one of four activity levels. The report defines those levels by splitting measured adults into quarters from least to most active (chapter 7), so "low active" is the second quarter, not a vague label. Picking your level is the single biggest judgement call, so treat the result as a starting estimate and confirm it against a couple of weeks of weigh-ins.

We also show a range around the estimate. The report publishes a standard error of about 339 calories a day for men and 246 for women; roughly two in three people like you fall within that band, so it is the range we display. The equations are defined for adults 19 and older, which is why the tools don't serve minors.

We deliberately don't use the common 1.2 to 1.9 "activity multipliers" that many calculators apply on top of a resting-metabolism formula. That multiplier table is a textbook convention we couldn't trace to a primary source, so instead we use equations that build activity in directly.

Resting burn (BMR)

Where we show resting metabolism on its own, we use the Mifflin-St Jeor equation:

BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5 (men) / − 161 (women)

It was built from 498 healthy adults aged 19 to 78. In a 2005 systematic review of the common prediction equations, Mifflin-St Jeor came within 10% of measured resting burn for more people than any other, which is why it is the usual recommendation. Its limits are worth stating: older adults and US ethnic minorities were underrepresented in the data it rests on, so for any individual it is still an estimate.

Weight projections

The year-long projections come from the NIH dynamic weight-change model (Hall et al., The Lancet 2011) — the same model behind the NIH Body Weight Planner. Unlike a fixed rule, it tracks fat, lean tissue, glycogen and body water separately and lets your energy burn fall as you lose weight, which is why its curve bends instead of running in a straight line.

We start the model at your estimated maintenance and then hold your intake and activity constant for the projection, with half of calories coming from carbohydrate. We solve the model's equations day by day with the standard fourth-order Runge-Kutta method. The constants come from the NIDDK model appendix published with the paper, and we cross-checked our starting body-composition equations against an open-source implementation, the bw package for R. Where that package adds the glycogen term in the wrong units, we follow the paper's own equation instead.

How we checked it

We verify the engine against published results, not just against itself. It reproduces the 2023 report's own worked example exactly: for a 22-year-old, low-active woman of 165 cm and 63 kg, it returns 2,275 calories a day, the figure the report prints. For the dynamic model, we use a validation case of a 100-kg man cutting about 478 calories a day (2 megajoules): the engine predicts 13.0 kg lost in the first year, against 22.6 kg from the 3,500-calorie rule, with half of the eventual loss reached in about 10 months and 95% in about 3.3 years — consistent with the paper's description of roughly a year and roughly three years. These checks run as automated tests.

The 3,500-calorie rule

The familiar "3,500 calories per pound" rule traces to Wishnofsky (1958), as described by Thomas et al. (2014) and Hall and Chow (2013). It assumes every pound lost costs a fixed 3,500 calories, which is a fair guide for the first few weeks but overstates loss over months, because it ignores the way your burn falls as you get lighter. We draw it next to the NIH model for comparison only, never as our own projection. For the detail, see how many calories are really in a pound.

Safety rules built into every tool

  • The tools are for adults 19 and older; younger ages are turned away, because the energy equations are defined for adults.
  • We don't plan intakes below 1,200 calories a day for women or 1,500 for men, the lowest levels Harvard Health Publishing advises without professional supervision.
  • Weight projections stop at the bottom of the healthy weight range for your height, and we don't project goals below it (CDC adult BMI categories).
  • The tools are not for pregnancy or breastfeeding, which change energy needs in ways these equations don't cover.

How the numbers stay correct

Every number on every page — in the tools, the worked examples and the written guides — is produced by the same calculation engine. A suite of automated tests recomputes each of those numbers from the engine on every change, so a page physically can't drift away from the math: if an equation and a page ever disagreed, the tests fail before anything ships. We round calories to the nearest 10 in results and weights to 0.1 lb, so small display differences are rounding, not disagreement.

Editorial policy

Our sources are government agencies, the National Academies and peer-reviewed research, and we cite them at the exact page or table used rather than in general. Every page shows the date it was last reviewed. If you spot an error, tell us through the contact page and we'll correct it. Ads on the site are served by Raptive and play no part in what we write or what the calculators compute.

Who is behind this

The site is researched, written and built by Marko Visic, MPharm, BSc Physics (University of Ljubljana). He is not a medical professional, and nothing here is medical advice — it's informational, and for personal decisions you should talk to your doctor or a registered dietitian. More on the about page.

Sources

Data as of October 2026. Reviewed October 7, 2026.