How Deficit Works: the math and the research

Deficit anchors every number on your lean body mass and shows its work. Below is each calculation the app and these tools run, why we chose it, and the research behind it. Where a number is our own calibration rather than a published equation, we say so plainly.

Estimates only. Talk to a doctor or registered dietitian before acting on these numbers for medical reasons.

The one rule: anchor on lean mass

Fat tissue burns almost no calories at rest and uses almost no dietary protein. Muscle does both. So the right number to build a cut on is your lean body mass, not your total weight. Resting burn follows from that, and so does protein once body fat is high, which is why two people at the same weight but different body fat get different targets. The calorie deficit calculator runs the full chain.

Resting metabolism: the Cunningham equation

What the research says.

Deficit estimates resting metabolic rate with the Cunningham equation (1991, the same numbers as Katch-McArdle): 

BMR = 370 + 21.6 × lean body mass (kg)

Cunningham's 1991 review of resting metabolism studies found that lean body mass explains 65 to 90% of the difference in resting burn between adults. Most calculators use the Mifflin-St Jeor equation instead, which leans on total weight, height, and age as stand-ins for lean mass. When you have a real body-fat number, Cunningham uses the actual signal rather than a proxy.

Maintenance: BMR × activity × digestion

Deficit's calibration (not a single published equation).

Maintenance calories come from three multiplied parts: 

TDEE = BMR × PAL × TEF

PAL is the physical-activity multiplier; TEF is the thermic effect of food, the energy spent digesting. Deficit sets PAL from your daily steps and your strength and sports workouts a week, from 1.20 to 1.55, and TEF from how you eat, from 1.08 to 1.15. Together they multiply BMR by 1.30 to 1.78. Many calculators go up to 1.9 for activity alone. This is a deliberate calibration, not a citation: a plan that starts a little low is easier to correct from your weekly trend than one that starts high. A long run or ride burns more than the activity level holds, so the app adds those calories on the day, as Fuel. The TDEE calculator shows the full breakdown.

How much protein should you eat on a cut?

What the research says.

Deficit sets protein from three things: your body fat, how you train, and how big your deficit is. For someone who lifts and is cutting, the range is 1.6 to 2.2 g per kg of body weight (0.73 to 1 g per lb). More workouts and a bigger deficit move you up the range, and a cut never gets less protein than maintenance would. Above 25% body fat for men or 32% for women, the target scales to lean mass instead (1.8 to 2.2 g per kg of lean mass), because fat does not need protein.

The ISSN position stand on protein and exercise (2017) finds 1.4 to 2.0 g/kg a day enough for most people who exercise. The Morton 2018 meta-analysis found no further muscle gain from training above about 1.6 g/kg a day. For lean lifters in a deficit, the Helms 2014 review suggests more: 2.3 to 3.1 g per kg of lean mass. The protein calculator shows where you land.

Body fat: how we estimate it

What the research says.

Body-fat percentage feeds the lean-mass number, so it has to come from somewhere. Deficit supports two at-home methods, both converted to a body-fat percentage with the Siri equation:

  • Tape measure. The U.S. Navy circumference method, from Hodgdon and Beckett (1984), with a typical error of about 3.5 points of body fat against underwater weighing.
  • Skinfold calipers. The Jackson-Pollock 3-site equations, sex-specific: men (1978) and women (1980).

The body fat percentage calculator runs both, plus an estimate from body type pictures.

How big should your calorie deficit be?

Deficit's calibration (informed by the research, not a single equation).

The recommended deficit is not a flat percentage. Deficit interpolates between body-fat anchor points: gentler when you are already lean (where there is least margin to lose muscle) and steeper when you have more fat to lose (where the body can pull more energy from fat stores). The anchors differ for men and women because essential fat is structurally higher in women. The Helms 2014 review supports the underlying rate, a weekly loss of roughly 0.5 to 1% of bodyweight to protect muscle; the specific anchor values are our calibration.

How long should a cut be?

Deficit's method.

A Deficit cut runs 8 weeks by default, with 4 to 12 supported. Long enough to show real body-composition change, short enough that adherence holds and metabolic adaptation stays modest. After the cycle you move to maintenance rather than dieting on indefinitely. The 8-week default is our choice; the principle that a cut should have an end date is the foundation of the app.

What these numbers can and can't do

Any equation-based estimate of metabolism sits within about ±10% of a person's measured value, and at-home body-fat methods have a typical error of about 3.5 to 4 points. That is accurate enough to start a cut, not to treat as exact. The point of the app is that it corrects the estimate weekly from your real weight and body-fat data, so the first week is calibration and every week after is measurement.

The underlying principle, that eating below your energy needs drives fat loss, is established public-health guidance (the Dietary Guidelines for Americans and NIH / NIDDK). Deficit's contribution is making it precise and personal, not inventing the rule.

These calculations are estimates for healthy adults training normally. They are not medical advice. Talk to a doctor or registered dietitian before acting on them for medical reasons, or if you are pregnant, have an eating disorder history, or manage a medical condition.

Sources

See it in the app

Deficit runs this math every week, automatically, and adjusts your calories, protein, and remaining cycle days based on what your body does.

Download on the App Store