How do I measure Original Gravity (OG) and Final Gravity (FG)?

Original Gravity (OG) is measured before pitching yeast, once your wort has cooled to room temperature. Final Gravity (FG) is measured after fermentation is complete — usually when gravity readings are stable over two consecutive days. Both are measured with a hydrometer (dropped into a sample cylinder) or a refractometer (a drop placed on the prism). Hydrometers are generally more accurate for FG since alcohol affects refractometer readings.

Why does the calculated ABV differ slightly from the actual ABV on the bottle?

The formula ABV ≈ (OG − FG) × 131.25 is a standard homebrewing approximation that works well for most beers in the typical 3–10% ABV range. It can deviate by ±0.1–0.3% from the true value. For very high-gravity beers (OG above 1.100), a more complex Brix-corrected formula is more accurate. Commercial breweries also use lab analysis (gas chromatography or ebulliometry) to certify exact ABV on labels, which is far more precise than any gravity-based formula.

Will diluting my beer affect the flavour, not just the ABV?

Yes, dilution affects much more than ABV. Adding water proportionally reduces hop bitterness (IBUs), malt flavour intensity, body, and colour. For highly hopped or strongly flavoured beers, even a 5–10% dilution can noticeably thin the body and soften bitterness. Brewers who routinely dilute high-gravity beers (a technique called "high-gravity brewing") account for this by over-hopping and over-malting the base batch. For homebrewers, it is worth doing a small test blend before diluting the full batch to check whether the result meets your expectations.

Homebrew Recipe & ABV Calculator

Recipe Details

Batch Size

Original Gravity (OG)

Final Gravity (FG)

Measure gravity with a hydrometer or refractometer. OG is measured before fermentation; FG after fermentation is complete.

Estimated Alcohol By Volume

5.25% ABV

1.050

1.010

Attenuation

80.0%

Formula: ABV ≈ (OG − FG) × 131.25. This is the standard homebrewing approximation (Fix & Fix, 1997). Actual ABV may vary ±0.1–0.3%.

About This Tool

Calculate estimated ABV from Original Gravity (OG) and Final Gravity (FG) using the standard homebrewing formula. Includes a dilution calculator to hit your target alcohol percentage. All calculations run in your browser — no login, no uploads.

ABV is calculated using the standard homebrewing approximation: ABV ≈ (OG − FG) × 131.25(Fix & Fix, 1997; Daniels, 1996). This formula is accurate to within ±0.1–0.3% for typical beers in the 3–10% ABV range. Measure specific gravity with a hydrometer or refractometer before and after fermentation.

How To Use

Enter your batch size — select litres (L) or gallons (gal) and type the volume.
Enter the Original Gravity (OG) — the specific gravity reading taken before pitching yeast (e.g. 1.050).
Enter the Final Gravity (FG) — the reading taken after fermentation is complete and stable (e.g. 1.010).
Click Calculate ABV — the estimated alcohol by volume appears instantly.
(Optional) Dilute to target ABV — open the dilution section and enter a target ABV to find out how many litres of water to add.

Example Calculations

Example 1: Standard Pale Ale — OG 1.050, FG 1.010

ABV = (1.050 − 1.010) × 131.25

ABV = 0.040 × 131.25

ABV ≈ 5.25%

Example 2: Dilution — lower a 5.25% ABV × 20 L batch to 4.5%

Final volume = (5.25 / 4.5) × 20 = 23.33 L

Water to add = 23.33 − 20 = 3.33 L

Dilution will proportionally reduce body and hop bitterness. A small test blend before diluting the full batch is recommended.

Frequently Asked Questions

How do I measure Original Gravity (OG) and Final Gravity (FG)?: Original Gravity (OG) is measured before pitching yeast, once your wort has cooled to room temperature. Final Gravity (FG) is measured after fermentation is complete — usually when gravity readings are stable over two consecutive days. Both are measured with a hydrometer (dropped into a sample cylinder) or a refractometer (a drop placed on the prism). Hydrometers are generally more accurate for FG since alcohol affects refractometer readings.
Why does the calculated ABV differ slightly from the actual ABV on the bottle?: The formula ABV ≈ (OG − FG) × 131.25 is a standard homebrewing approximation that works well for most beers in the typical 3–10% ABV range. It can deviate by ±0.1–0.3% from the true value. For very high-gravity beers (OG above 1.100), a more complex Brix-corrected formula is more accurate. Commercial breweries also use lab analysis (gas chromatography or ebulliometry) to certify exact ABV on labels, which is far more precise than any gravity-based formula.
Will diluting my beer affect the flavour, not just the ABV?: Yes, dilution affects much more than ABV. Adding water proportionally reduces hop bitterness (IBUs), malt flavour intensity, body, and colour. For highly hopped or strongly flavoured beers, even a 5–10% dilution can noticeably thin the body and soften bitterness. Brewers who routinely dilute high-gravity beers (a technique called "high-gravity brewing") account for this by over-hopping and over-malting the base batch. For homebrewers, it is worth doing a small test blend before diluting the full batch to check whether the result meets your expectations.

This tool is provided for informational purposes only. Results are estimates and should not be relied upon as professional advice.

Related Tools

BAC Calculator (Blood Alcohol Concentration)

Estimate your Blood Alcohol Concentration (BAC) using the Widmark formula. Enter your sex, weight, drinks consumed, and time elapsed to get an instant estimate — for informational reference only. Never use this to judge fitness to drive.