# 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.

- Calculator id: `calorie-deficit` · Category: Health (`health`) · Tool name: `calculate_calorie_deficit_timeline`
- Canonical page: https://tttkmbb.com/health/calorie-deficit · This document: https://tttkmbb.com/health/calorie-deficit.md · JSON definition: https://tttkmbb.com/health/calorie-deficit.json

## Purpose

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.

**Use when:** 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.

## Input

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

## Output

| Field | 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. |

## 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).

## Data Sources

- Hall KD et al. (2011) Quantification of the effect of energy imbalance on bodyweight, Lancet 378:826-837 — https://pubmed.ncbi.nlm.nih.gov/21872751/ (peer_reviewed, retrieved 2026-09-23)
- NIH Body Weight Planner — https://www.niddk.nih.gov/bwp (reference, retrieved 2026-09-23)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/calorie-deficit?current_weight_kg=…&target_weight_kg=…`
- `POST https://tttkmbb.com/api/v1/calculate/calorie-deficit` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/calorie-deficit · OpenAPI operationId `calculate_calorie_deficit_timeline` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "calorie-deficit", "inputs": {…}}`

## Example

- 80 → 75 kg at 500 kcal/day: inputs `{"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
```

## Limitations

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. 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). All values are computed from the formula above; no measurement or live data is involved.

## 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

- [TDEE Calculator](https://tttkmbb.com/health/tdee.md) — Find maintenance calories to set the deficit from.