# Final Gravity Calculator

Final gravity (FG) is the specific gravity of beer after fermentation is complete, predicted from the original gravity and the yeast strain's apparent attenuation. The formula is FG = 1 + (OG − 1) × (1 − attenuation / 100). Mash temperature, grain bill composition and pitch rate all shift the result, typically within a ±5 percentage point window around the yeast's published figure.

Yeast strains publish an apparent attenuation range. That, plus your original gravity, is enough to know roughly where the beer will finish.

HTML version: https://brewgravity.com/calculators/final-gravity/

## Inputs

- **Original gravity (OG)** (min 1, step 0.001) — default: 1.055
- **Yeast apparent attenuation (%)** (min 0, max 100, step 1) — default: 75

## Outputs

- **fg**: Predicted FG
- **abv**: Predicted ABV
- **range**: Likely range

## FAQ

### My beer finished higher than predicted. Why?

Most often a high mash temperature, a lot of crystal malt, or an underpitched or cold fermentation. Warming the fermenter a couple of degrees and rousing the yeast will often restart it if it has stalled.

### How do I know when fermentation is actually finished?

Two identical gravity readings taken three days apart. Airlock activity tells you nothing useful — it stops long before the yeast does, and it starts again if the temperature changes.

### Where do I find my yeast's attenuation?

The manufacturer publishes it on the packet or the strain page, usually as a range. Brewgravity has it built in for the strains in its database, so choosing a yeast fills the figure in for you.

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Source: https://brewgravity.com — Free final gravity calculator predicting FG and ABV from original gravity and yeast apparent attenuation, with typical attenuation ranges by yeast type and the factors that shift them.