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Calorie Deficit & Weight Loss Time Calculator

Converts a weight change goal into total energy and the number of days and weeks required at a given daily calorie deficit or surplus, using 7,700 kcal per kilogram of body fat.

When to use

You know the current and target weight and a planned daily deficit (or surplus) and want a time estimate, or want the deficit needed to hit a date.

Do not use when: You need a medically supervised plan, the target implies more than 1 kg/week loss, or the person is a child; metabolic adaptation makes long projections approximate.

Formula

total_kcal = |target − current| × 7700; days = total_kcal / daily_deficit; weekly_change = daily_deficit × 7 / 7700

The 7,700 kcal/kg (3,500 kcal/lb) rule is a linear approximation; real loss slows as body mass and BMR fall (see Hall et al. 2011 for dynamic models).

Inputs

Parameter	Type	Unit	Required	Description
`current_weight_kg`	number	kg	yes	Current body weight in kilograms. Range: > 0, ≤ 700
`target_weight_kg`	number	kg	yes	Goal body weight in kilograms. Range: > 0, ≤ 700
`daily_deficit_kcal`	number	kcal/day	default 500	Absolute daily calorie deficit (for loss) or surplus (for gain). Sign is inferred from the weight change. Range: ≥ 50, ≤ 2000

Outputs

Output	Type	Unit	Description
`weight_change_kg`	number	kg	Target minus current weight (negative = loss).
`total_energy_kcal`	number	kcal	Energy equivalent of the weight change at 7,700 kcal/kg.
`days`	number	days	Days needed at the given daily deficit/surplus.
`weeks`	number	weeks	Same duration in weeks.
`weekly_change_kg`	number	kg/week	Rate of weight change implied by the deficit.

Example

80 → 75 kg at 500 kcal/day: {"current_weight_kg":80,"target_weight_kg":75,"daily_deficit_kcal":500} → {"weight_change_kg":-5,"total_energy_kcal":38500,"days":77,"weeks":11,"weekly_change_kg":-0.45}

GET https://tttkmbb.com/api/v1/calculate/calorie-deficit?current_weight_kg=80&target_weight_kg=75&daily_deficit_kcal=500

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/calorie-deficit (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/calorie-deficit · Markdown: https://tttkmbb.com/health/calorie-deficit.md · JSON definition: https://tttkmbb.com/health/calorie-deficit.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="calorie-deficit"
OpenAPI operationId: calculate_calorie_deficit_timeline
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Hall KD et al. (2011) Quantification of the effect of energy imbalance on bodyweight, Lancet 378:826-837 (peer_reviewed)
NIH Body Weight Planner (reference)

FAQ

Is 500 kcal/day safe?

For most adults a 500 kcal deficit (≈0.45 kg/week) is the commonly recommended upper-moderate rate; do not go below about 1,200 kcal/day (women) or 1,500 kcal/day (men) without supervision.

Why is my real loss slower than predicted?

Energy expenditure drops as you lose weight and adherence is rarely perfect. Expect the linear estimate to be optimistic by 20–30% over several months.

Related calculators

TDEE Calculator — Find maintenance calories to set the deficit from.