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Alcohol by volume calculator

An original gravity of 1.050 and a final gravity of 1.010 mean 5.25% alcohol by volume. Enter your two hydrometer readings for beer, cider, mead or wine and you also get the gravity drop and the apparent attenuation. Free in your browser.

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Worked example

If you enter

Original gravity: 1.05
Final gravity: 1.01

You get

Gravity drop: 0.04
Gravity points og: 50
Gravity points fg: 10
Gravity points drop: 40

Homebrewers, cider makers, mead makers, and small wineries need a single alcohol percentage from two hydrometer numbers: original gravity taken before yeast starts work, and final gravity taken when fermentation has finished or been stopped. This alcohol by volume calculator accepts those specific-gravity readings, applies the standard brewing identity ABV equals original gravity minus final gravity times one hundred thirty-one point two five, and returns the percentage together with the gravity drop and, when original gravity sits above 1.000, the apparent attenuation. A pale-ale pair of 1.050 and 1.010 yields 5.25 percent alcohol by volume; a stuck or unfermented batch with equal readings yields zero.

What your two readings say about alcohol

Alcohol by volume from hydrometer readings is the percentage of ethanol in a finished beer, cider, mead, or wine estimated from how much sugar density disappeared during fermentation. Original gravity is the specific gravity of wort, must, or juice before yeast consumes fermentable extract; final gravity is the specific gravity after fermentation has finished or been halted. Because ethanol is less dense than the sugar solution it replaces, the reading falls, and the size of that drop is a practical proxy for alcohol.

The identity used here is the standard brewing formula: alcohol by volume equals original gravity minus final gravity, multiplied by one hundred thirty-one point two five. That maps a specific-gravity pair such as 1.050 and 1.010 onto 5.25 percent alcohol by volume, which is the figure homebrew logs, club competition sheets, and kitchen recipe cards actually quote.

Apparent attenuation is reported when original gravity is greater than 1.000 as the drop divided by the extract above water, times one hundred, so a drop from 1.050 to 1.010 is eighty percent apparent attenuation. Gravity points, which are (specific gravity minus one) times one thousand, are echoed for both readings and for the drop so a log that stores 50 and 10 instead of 1.050 and 1.010 can still audit the same pair.

The calculator does not convert Plato, Brix, or gravity-point shorthand: send specific gravity on the hydrometer scale around 1.000. It also does not apply sample-temperature corrections or the more elaborate high-gravity alternate formula; those are separate laboratory steps.

What you can do with it

Package-day beer and cider logs

Enter original gravity from brew day and final gravity from the fermenter to record ABV % on bottles, kegs, or a club competition sheet.

Catch swapped hydrometer readings

Reject a log line where final gravity exceeds original gravity instead of storing a negative alcohol percentage.

Deterministic keys for brew software

Seed recipe importers and teaching worksheets with the standard (OG − FG) × 131.25 identity so every client prints the same ABV.

FAQ

What happens if final gravity is higher than original gravity?

Fermentation consumes extract and cannot raise density, so a higher finishing reading is treated as a swapped or mistyped pair and is not billed.

Can I enter Plato degrees or gravity points instead of specific gravity?

No. Both fields must be hydrometer specific gravity between 0.980 and 1.200 (for example 1.050 and 1.010). Plato, Brix, and point shorthand such as 50 are rejected.

What ABV do I get when original gravity equals final gravity?

Zero percent. Equal readings mean no extract was consumed, which is the honest result for an unpitched or fully stuck batch.

What is apparent attenuation?

It is the share of the original extract above water that fermentation used up. A drop from 1.050 to 1.010 is 80% apparent attenuation, and it is shown whenever the original gravity sits above 1.000.

Does the calculator correct readings for sample temperature?

No. It takes the two readings as you enter them and applies the standard formula, without temperature corrections or the alternative formula some brewers use for very strong batches. Those are separate laboratory steps.

Related tools

For developers — API access

Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.

API endpoint

POSThttps://api.kit.forhosting.com/cook/alcohol-by-volume

Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.

Call it from your stack

curl -X POST https://api.kit.forhosting.com/cook/alcohol-by-volume \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"original_gravity":1.05,"final_gravity":1.01}'

const res = await fetch("https://api.kit.forhosting.com/cook/alcohol-by-volume", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "original_gravity": 1.05,
    "final_gravity": 1.01
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/cook/alcohol-by-volume",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "original_gravity": 1.05,
    "final_gravity": 1.01
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/cook/alcohol-by-volume", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"original_gravity":1.05,"final_gravity":1.01}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"original_gravity":1.05,"final_gravity":1.01}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/cook/alcohol-by-volume", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "original_gravity": 1.05,
  "final_gravity": 1.01
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "cook.alcohol_by_volume",
  "status": "queued",
  "_links": {
    "result": "/tasks/tsk_…/result"
  }
}

The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.

Pricing

Per request$0.002

Published price — no tokens, no invented credits. A failed task is never charged.

Errors

HTTP	Code	Meaning
`401`	`unauthorized`	Missing or invalid API key.
`402`	`insufficient_balance`	Your balance doesn't cover the task price.
`404`	`unknown_type`	That task type doesn't exist.
`429`	`rate_limited`	Too many requests. Use the webhook instead of polling.

Technical notes

If the final gravity is higher than the original gravity the task fails with invalid_input, because fermentation consumes extract and cannot raise density above the starting wort, must, or juice.

What alcohol by volume from gravity means and what the response contains

If final gravity exceeds original gravity the request is rejected, because a ferment cannot raise density above the starting reading.

Input contract, formula, range checks, and error behaviour

Both readings must sit between 0.980 and 1.200 inclusive, which is the practical hydrometer window for beer, cider, mead, and table wine. Equal readings are valid and return zero percent alcohol by volume, which is the honest result for an unpitched or fully stuck batch. POST a JSON body with two required finite numbers: original_gravity, the hydrometer specific gravity of the unfermented wort, must, or juice; and final_gravity, the hydrometer specific gravity of the finished or halted beer, cider, mead, or wine. Each field may arrive as a JSON number or as a numeric string; aliases og, original, og_sg, start_gravity, and og_reading map to original_gravity, and aliases fg, final, fg_sg, finishing_gravity, and fg_reading map to final_gravity, so spreadsheets and typed forms can reuse the same contract. Values such as 12 (Plato) or 50 (gravity points) fail with invalid_input naming the field and stating that a specific gravity such as 1.050 is required. Missing fields, empty strings, non-finite numbers, and out-of-range readings also fail with invalid_input and are not billed. On success the body echoes both gravities, reports gravity_drop, gravity_points_og, gravity_points_fg, and gravity_points_drop, returns abv as the percentage, includes apparent_attenuation_pct when original gravity is greater than 1.000, and restates the formula abv equals (original_gravity minus final_gravity) times 131.25 with method standard_131_25.

Where an alcohol by volume calculator earns its keep

Home brewers meet this calculation at packaging day, when a hydrometer in the fermenter and the original brew-day reading must become a label percentage for bottles, kegs, or a club competition form. Cider and mead makers use the same identity on juice and honey musts whose original gravity often sits higher than beer, and they need a deterministic oracle that still accepts a final gravity below 1.000 when a very dry ferment finishes under water. Small wineries and teaching labs check student hydrometer pairs against a fixed coefficient instead of opaque phone apps that silently switch to Plato or apply an unpublished high-gravity correction. Culinary schools and beverage programs use the endpoint as a worksheet key: original 1.050 and final 1.010 must always yield 5.25 percent, not a rounded guess that drifts between platforms. On this page the work runs entirely in your browser; through the API each successful lookup costs $0.002, failed validation is not charged, and nothing is stored after the response returns. That split—free interactive exploration and a priced, identical API—lets you move from a kitchen hydrometer demo to an automated brew-log pipeline without changing the mathematical result. Pair it with unit converters when you only have Plato or Brix, and keep this endpoint when the search intent is specifically alcohol by volume from original gravity and final gravity.

Technical questions

How is alcohol by volume calculated from original gravity and final gravity?

Send specific-gravity readings original_gravity and final_gravity. The API returns ABV% = (OG − FG) × 131.25, plus gravity drop, gravity points, and apparent attenuation when OG is greater than 1.000.

What does the alcohol by volume calculator cost?

The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.

Read the full KIT documentation →

Run — free