Original Gravity Calculator

ByChef Claire Delacroix, MS ✓ Verified 📚 151 Calculators Authored

Food Scientist & Culinary Chemistry Specialist · M.S. in Food Science and Technology, Certified Culinary Scientist (CCS)

📅 Published: Sep 28, 2026

The formula

ABV ≈ (OG − FG) × 131.25; attenuation = (OG−FG) ÷ (OG−1) × 100

What the original gravity tells you

The hydrometer maths behind original gravity: ABV, apparent attenuation and points consumed from two gravity readings.

Calculation: ABV ≈ (OG − FG) × 131.25; attenuation = (OG−FG) ÷ (OG−1) × 100. Enter your inputs and every substitution is shown below the result.

Reading the result

90 %+ attenuation reads dry; under 70 % is sweet or stuck — the attenuation band diagnoses as much as the ABV.

Limitations and best practice

The 131.25 factor loses a few tenths above 8 % ABV where alcohol density effects grow.

This calculator gives an estimate based on standard formulas; it is not professional financial, medical, legal or engineering advice. Verify anything you plan to act on with a qualified professional and your own records.

Frequently asked questions

Why an estimate?

Distillation-free lab methods (density + refractive) exist; the (OG−FG)×131.25 is the accepted homebrew standard with <0.3 % typical error.

Can I read FG too early?

Confirm terminal gravity over three days before calling it; attenuation hides in stalled ferments.

Source & methodology

Standard brewing arithmetic (De Clerck).

Original Gravity explained: formula, worked example and honest limits

This page was written for one job: turning your own inputs into a defensible figure, with the working shown. Original Gravity is the kind of number people argue about from memory. This page ends that argument with arithmetic you can watch happen: enter your figures, get Estimated ABV in real time, and see every substitution printed below.

What makes Original Gravity worth calculating is not precision but position: it tells you which side of a threshold you are on, and by how much. A rough figure that settles the direction beats an elaborate one that never commits.

There is a habit that separates people who get useful answers from Fermentation numbers: they decide what question the figure answers before they compute it. Original Gravity answers “Estimated ABV?” — and nothing else, which is precisely why the result is trustworthy.

What makes a calculation page trustworthy is that its copy is derived from its own arithmetic: here estimated ABV = 4.99 %, recomputed every time a field changes, and the sensitivity section ranks the inputs that move the result — Original gravity first, at ≈275.8 % of the headline for a ten-percent overstatement of that input — which is the thread the rest of this guide pulls on.

The Original Gravity formula, explained

Here is the complete model this tool runs — ABV ≈ (OG − FG) × 131.25; attenuation = (OG−FG) ÷ (OG−1) × 100 — with each input explained below. If a competitor’s page gives you a different answer from the same inputs, this is the text that settles which of you is right.

Original gravity (example value 1) is a plain measured quantity — the unit shown in the label is the unit the formula expects. The example value is there to demonstrate the mechanics, not to describe your situation — overwrite it.

Final gravity (example value 1) is a measured number in the unit named by the label; the formula does not convert for you. This input feeds the formula directly; a defensible figure here is worth more than false precision anywhere else.

Original Gravity with real numbers, step by step

Abstract formulas are hard to trust, so here is the complete substitution using the default inputs, written the way an examiner would want it. Starting figures: Original gravity, Final gravity. Run to the end, the estimated ABV resolves to 4.99 % — Standard table beer range on the bands this tool uses.

Δ gravity = 1.048 − 1.010 = 0.038
ABV = × 131.25 = 4.99 %
Attenuation = Δ ÷ OG-points = 79.2 %

Attenuation — 79.2%. If your inputs change, this is usually the first companion figure to move.

Points consumed — 38.0 pts. If your inputs change, this is usually the first companion figure to move.

Real extract left (approx) — 10.0 °P. Compare it against the main result of 4.99 % before drawing conclusions.

Re-running with your own figures is instant; re-running by hand once is the best way to know the tool is telling you the truth.

How far Original Gravity can reasonably move

Single-point answers invite false confidence, so the same engine was run across a band around the input that matters most here. Each row is a full recalculation of Original Gravity, not an interpolation. Settings beyond this model’s valid range were dropped rather than guessed at, which is why the table is shorter on one side.

Original gravity setting	Estimated ABV
example inputs	4.99 %
110 % of the example value	18.74 %
120 % of the example value	32.50 %

Across 3 settings of Original gravity, the result spans 4.99 % to 32.50 %. A decision that only works at one row of this table is a decision waiting for an input to move. Reproduce it in the form above by editing Original gravity alone; the substitution steps will follow your figures exactly.

Where Original Gravity is fragile — and where it is not

This list was computed for this engine just now: ten percent on one input at a time, everything else on its example value. Some fields can only be tested in one direction here, and those rows say so instead of inventing a second figure. It is the fastest honest answer to “what matters most in Original Gravity?” — and it is specific to these defaults, because sensitivity moves with the starting point.

Original gravity +10 % → 18.74 % against 4.99 % at the example inputs (≈275.8 %); the opposite direction leaves the model’s valid range, so it was not computed
Final gravity −10 % → 18.24 % against 4.99 % at the example inputs (≈265.8 %); the opposite direction leaves the model’s valid range, so it was not computed

Original gravity is the dominant lever here: a ten-percent increase moves the result by about 275.8 %, from 4.99 % at the example value up to 18.74 %. Spend your verification effort there first, and Final gravity is effectively tied with it (≈265.8 % for the same nudge), so verify the pair together. Everything below that can stay approximate, which is a licence to stop chasing data that would not change the decision.

Reading the result: bands, zones and thresholds

A number on its own is trivia; a number against a range is intelligence. The verdict attached to the default calculation — “Standard table beer range” — is one band on a ladder your own figure may sit somewhere else on. How much of that band is luck? On Original gravity alone a ten-percent increase covers 4.99 % at the example value up to 18.74 % — the opposite direction leaves this model’s valid range, so only the upper probe is reported. The practical reading rules: ask what would have to change to move you one band. If the answer is “a small, plausible input difference”, the band itself is not the finding — the uncertainty is.

The comparison worth making is not with somebody else’s Original Gravity but with the version of your own situation measured last quarter — same formula, same conventions, different inputs.

Why Original Gravity results go wrong — and how to prevent it

The 131.25 factor loses a few tenths above 8 % ABV where alcohol density effects grow. This calculator gives an estimate based on standard formulas; it is not professional financial, medical, legal or engineering advice. Verify anything you plan to act on with a qualified professional and your own records.

Stopping at the first plausible answer. If the result surprises you, the productive question is which input would have to be wrong for the surprise to disappear — then go check that input.

This model has its own particular trap: Original gravity carries ≈275.8 % of the response per ten percent of error, so effort spent elsewhere is largely decorative.

A final trap is presentation: reporting Original Gravity to more decimal places than the inputs support. The tool prints what the arithmetic gives; you should quote only what your least precise field justifies.

Original Gravity verification checklist

Note the date and the inputs beside the number so the figure can be audited later.
Confirm the example values are plausible for your own situation before replacing them.
Re-verify Original gravity against a source document — ten percent of error there moves the headline by ≈275.8 %.
Re-run the calculation with a ±10 % stress on the two most sensitive inputs.
State, in one sentence, which convention each rate, unit or period in the form is using.
Run the ±10 % stress on the top two levers and check that your conclusion survives both.

How to use the Original Gravity tool properly

The tool does the arithmetic; the routine below is what makes the arithmetic trustworthy. Five steps, a minute total — and afterwards you will know not just what the number is, but which assumption produced it.

Step 1 — one document, one pass. Open the statement, label or record that holds original gravity, final gravity and transcribe straight into the form. Transcription from a single source removes the mismatch errors that appear when figures are assembled from three different places. If a result feels obviously right, that is the moment to check the inputs hardest.

Step 2 — start from the case you can defend. Enter pessimistic-but-honest figures first; the optimistic run is a ceiling, not a plan. A conclusion that holds on conservative inputs holds everywhere. A figure nobody can reproduce is a figure nobody will act on twice.

Step 3 — play with the inputs on purpose. Ten percent up, ten percent down, one field at a time. Within a few minutes you will know this calculation better than most people who quote its results. Comparisons are cheap: run the identical calculation for the alternative and look at the gap.

Step 4 — run the scenario trio. Expected, pessimistic, optimistic. The gap between pessimistic and expected is the number to plan against; the optimistic result is useful only as a ceiling. A result worth acting on survives having its two shakiest inputs halved.

Step 5 — record the context, not just the number. Note today’s date and the inputs alongside the 4.99 % output. Six months from now the context is what turns this figure into a trend. Re-running a number costs seconds; retracting a decision costs considerably more.

Where Original Gravity matters in practice

Most Fermentation disagreements are arithmetic disagreements in disguise. When Estimated ABV of 4.99 % sits on the table with its working shown, the conversation stops being about who sounds more confident and starts being about which input is wrong. Comparisons are cheap: run the identical calculation for the alternative and look at the gap.

Used properly this is a five-minute filter that saves weeks. Compute Original Gravity, reject what fails, and only then bring in the expensive analysis for the survivors. The point of transparency is that a reader can disagree with a specific line rather than with you.

Treat Original Gravity like a smoke alarm rather than a painting — its value is in being tested periodically, not in being admired once. Compare the calculation to the rough guess you arrived with, and ask why they differ.

Key terms on this page

Attenuation: 79.2% in the default Original Gravity calculation — one of the secondary figures an experienced reader checks before trusting the primary number.
Points consumed: 38.0 pts for the Original Gravity worked case; it sits next to the main result because it carries context the headline leaves out.
Real extract left (approx): 10.0 °P for the Original Gravity worked case; it sits next to the main result because it carries context the headline leaves out.
Estimated ABV: The primary output of this tool: 4.99 % for the default inputs, computed exactly from ABV ≈ (OG − FG) × 131.25; attenuation = (OG−FG) ÷ (OG−1) × 100.
Worked example: The step-by-step substitution printed above, so the arithmetic can be re-derived by hand or in a spreadsheet.
Verdict band: The plain-language label — here “Standard table beer range” — mapping the computed figure onto experience-based ranges for Cooking & Food Science.
Sensitivity (+10 %): How far the Original Gravity moves when a single input is nudged ten percent, in whichever direction the model accepts; the ranked list on this page is computed live from this engine.

The provenance of this Original Gravity calculation

Everything above runs on one stated formula, applied without hidden coefficients: ABV ≈ (OG − FG) × 131.25; attenuation = (OG−FG) ÷ (OG−1) × 100. Source of record: Standard brewing arithmetic (De Clerck). Any other implementation of this model should return the same figure for the same inputs. If one does not, the difference is in its assumptions — which this page prints and that one probably does not.

Treat the output as a precise answer to the question you typed: exact arithmetic, transparent formula, published source, no hidden rounding. The sensitivity table above is part of that promise rather than a bonus: it lets you see how much of the result depends on how carefully you measured.

Turning this Original Gravity into a decision

Save the number together with its inputs and today’s date; an Original Gravity figure without context is unusable the moment an input changes.

This page was written for people who need one trustworthy number before their next decision, and the promise is that nothing above required trust. Bookmark the tool, re-run it whenever an input moves by more than a few percent, and keep the date with the number.