Deficit

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. Every calculation here follows from that, which is why two people at the same weight but different body fat get different targets. The lean body mass calculator runs the full chain; the protein anchor post works through why bodyweight breaks at the edges.

Resting metabolism: the Cunningham equation

What the research says.

Deficit estimates resting metabolic rate with the Cunningham equation: 

BMR = 500 + 22 × lean body mass (kg)

Cunningham reanalyzed the classic metabolic data and found lean body mass to be the single best predictor of resting metabolism, with sex and age adding little once lean mass is known. 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. For how it compares to the other BMR equations, see the Cunningham equation for athletes.

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 uses a tighter PAL range (1.10 to 1.40) and an explicit TEF range (1.08 to 1.15) than the generic 1.2 to 1.9 multipliers found elsewhere. This is a deliberate calibration, not a citation: because Cunningham returns a BMR roughly 10% higher than Mifflin-St Jeor, the activity multiplier comes down to match, so the final TDEE lands in the same place but on a more body-composition-aware base. The TDEE calculator shows the full breakdown, and how Deficit calculates your calories walks the same pipeline step by step.

How much protein should you eat on a cut?

What the research says.

Deficit sets protein at 2.2 g per kg of lean body mass. The ISSN position stand on protein and exercise (2017) and the Helms 2014 review on natural bodybuilders in a deficit both land near the top of that range, and the Morton 2018 meta-analysis finds returns flatten beyond it. Below about 1.8 g/kg lean mass, muscle retention suffers in a deficit; above about 2.4, no controlled study shows added benefit. So 1.8 to 2.2 is the protective band, and 2.2 is the practical ceiling worth eating to on a cut. Anchoring on lean mass rather than bodyweight is the whole argument here.

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:

The body fat percentage calculator runs both and explains the accuracy tradeoffs.

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 within about ±3%. 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.

Try for free