# Homebrew ABV Calculator

> Computes a beer's alcohol by volume from original and final specific gravity with the standard (OG − FG) × 131.25 formula and the alternate high-gravity formula, plus alcohol by weight, apparent attenuation, degrees Plato, real extract and calories per serving.

- Calculator id: `homebrew-abv` · Category: Food & Cooking (`food`) · Tool name: `calculate_homebrew_abv`
- Canonical page: https://tttkmbb.com/food/homebrew-abv · This document: https://tttkmbb.com/food/homebrew-abv.md · JSON definition: https://tttkmbb.com/food/homebrew-abv.json

## Purpose

Computes a beer's alcohol by volume from original and final specific gravity with the standard (OG − FG) × 131.25 formula and the alternate high-gravity formula, plus alcohol by weight, apparent attenuation, degrees Plato, real extract and calories per serving.

**Use when:** You have hydrometer readings (or refractometer readings corrected for alcohol) before and after fermentation and want the alcohol content, attenuation or calorie content of a homebrewed beer, cider or mead.

**Do not use when:** You need the strength of a mixed drink (use cocktail-abv) or the readings come from an uncorrected refractometer after fermentation (alcohol distorts Brix readings). Results are informational and must not be used to decide whether someone is fit to drive.

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `original_gravity` | number | SG | required | Specific gravity of the wort or must before fermentation, e.g. 1.050. (min 1.001, max 1.2) |
| `final_gravity` | number | SG | required | Specific gravity after fermentation, e.g. 1.010; must be lower than the original gravity. (min 0.98, max 1.1) |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `abv_standard_percent` | number | % | Alcohol by volume = (OG − FG) × 131.25. |
| `abv_alternate_percent` | number | % | Alcohol by volume from the non-linear formula 76.08 × (OG − FG) / (1.775 − OG) × FG / 0.794, more accurate above about 6–7 % ABV. |
| `abw_percent` | number | % | ABW = ABV × 0.789 / FG (ethanol density 0.789 g/ml relative to the beer's density). |
| `apparent_attenuation_percent` | number | % | (OG − FG) / (OG − 1) × 100; typical ale and lager yeasts reach 65–85 %. |
| `original_plato` | number | °P | Original gravity converted to degrees Plato (cubic SG→°P fit). |
| `final_plato` | number | °P | Final gravity converted to degrees Plato (apparent extract). |
| `real_extract_plato` | number | °P | Balling: 0.1808 × original °P + 0.8192 × apparent final °P (residual sugars after correcting for the lighter alcohol). |
| `real_attenuation_percent` | number | % | (original °P − real extract) / original °P × 100. |
| `calories_per_12oz` | number | kcal | Energy from alcohol and residual extract in a 355 ml serving. |
| `calories_per_100ml` | number | kcal | Same energy expressed per 100 ml (12 US fl oz = 354.88 ml). |

## Formula

`ABV = (OG − FG) × 131.25; ABV_alt = 76.08 × (OG − FG) / (1.775 − OG) × FG / 0.794; ABW = ABV × 0.789 / FG; apparent attenuation = (OG − FG) / (OG − 1) × 100; °P = −616.868 + 1111.14·SG − 630.272·SG² + 135.997·SG³; real extract = 0.1808·OP + 0.8192·AP; real attenuation = (OP − RE) / OP × 100; kcal per 12 oz = 1881.22·FG·(OG − FG) / (1.775 − OG) + 3550·FG·(0.1808·OG + 0.8114·FG − 1); kcal per 100 ml = kcal per 12 oz / 3.5488`

The 131.25 factor is accurate to about ±0.5 % ABV for beers up to roughly 7 %; the alternate formula corrects for the non-linear relation between gravity drop and alcohol at higher gravities. The calorie formula (alcohol plus residual extract) is the one used by homebrewing references and does not account for unfermentable additions. Informational only: results must not be used to decide whether anyone is fit to drive.

## Data Sources

- Wikipedia – Alcohol by volume (ABV from gravity difference; ABV/ABW relation) — https://en.wikipedia.org/wiki/Alcohol_by_volume (reference, retrieved 2026-09-24)
- Wikipedia – Beer measurement (specific gravity, degrees Plato, IBU, SRM/EBC) — https://en.wikipedia.org/wiki/Beer_measurement (reference, retrieved 2026-09-24)
- Brewer's Friend – ABV calculator (standard and alternate ABV formulas, calorie formula) — https://www.brewersfriend.com/abv-calculator/ (reference, retrieved 2026-09-24)

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/homebrew-abv?original_gravity=…&final_gravity=…`
- `POST https://tttkmbb.com/api/v1/calculate/homebrew-abv` 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/homebrew-abv · OpenAPI operationId `calculate_homebrew_abv` 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": "homebrew-abv", "inputs": {…}}`

## Example

- OG 1.050, FG 1.010: inputs `{"original_gravity":1.05,"final_gravity":1.01}` → `{"abv_standard_percent":5.25,"abv_alternate_percent":5.34,"apparent_attenuation_percent":80,"original_plato":12.39,"final_plato":2.56,"calories_per_12oz":138}`
- OG 1.080, FG 1.020 (strong ale): inputs `{"original_gravity":1.08,"final_gravity":1.02}` → `{"abv_standard_percent":7.88,"abv_alternate_percent":8.44,"apparent_attenuation_percent":75,"original_plato":19.33,"calories_per_12oz":249}`

```
GET https://tttkmbb.com/api/v1/calculate/homebrew-abv?original_gravity=1.05&final_gravity=1.01
```

## Limitations

You need the strength of a mixed drink (use cocktail-abv) or the readings come from an uncorrected refractometer after fermentation (alcohol distorts Brix readings). Results are informational and must not be used to decide whether someone is fit to drive. The 131.25 factor is accurate to about ±0.5 % ABV for beers up to roughly 7 %; the alternate formula corrects for the non-linear relation between gravity drop and alcohol at higher gravities. The calorie formula (alcohol plus residual extract) is the one used by homebrewing references and does not account for unfermentable additions. Informational only: results must not be used to decide whether anyone is fit to drive. All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**Which ABV formula should I report?**

Use the standard formula for ordinary beers; it is what most competitions and recipe software use. Above about 1.070 OG the alternate formula is closer to laboratory values because the simple linear factor under-reads.

**Do I need to temperature-correct my hydrometer readings?**

Yes: most hydrometers are calibrated at 15.6 or 20 °C. Take both readings at the calibration temperature or apply the same correction to each before entering them.

**Why does the final gravity read below 1.000 sometimes?**

Alcohol is lighter than water, so a very dry beer, cider or mead can finish at 0.990–0.998; the calculator accepts final gravities down to 0.980.

## Related

- [Priming Sugar Calculator](https://tttkmbb.com/food/priming-sugar.md) — Bottle-condition the finished beer to a target carbonation.
- [IBU Calculator (Tinseth)](https://tttkmbb.com/food/ibu-tinseth.md) — Bitterness of the same recipe from the hop additions.
- [Cocktail ABV Calculator](https://tttkmbb.com/food/cocktail-abv.md) — Alcohol content of mixed drinks.