Here is the calculation this page runs, in full, before anything else: a 250 cc dual-sport single with a 76 mm bore and a 55 mm stroke. π ÷ 4 × 76² × 55 = 249,500 cubic millimetres. Divide by 1,000 and you have 249.5 cc — which is exactly why the badge reads "250". Enter your own bore and stroke in millimetres above and the panel does the same three steps for any engine, single or multi-cylinder.
Working in millimetres, step by step
Metric displacement has one extra move that trips people up. You compute π ÷ 4 × bore² × stroke with both figures in millimetres, which gives cubic millimetres — and there are 1,000 cubic millimetres in a cubic centimetre, so you divide by 1,000 to reach cc. Then multiply by the cylinder count. Skip the divide-by-1,000 and your answer is a thousand times too big; it is the single most common metric mistake, and the calculator handles it for you.
From one cylinder to the whole engine
The formula gives one cylinder's swept volume; the engine's displacement is that multiplied by how many cylinders it has. A 600 cc supersport four is about 150 cc per cylinder; a 250 cc parallel twin is about 125 cc per pot. The panel shows both the per-cylinder cc and the total, so you never confuse one cylinder for the engine.
Which class does your cc land in?
As soon as the tool has a total, it names the class — because a metric figure usually matters for a reason: a licence category, an insurance bracket, a racing group. The common bands look like this:
| Class | Typical use |
|---|---|
| 50 cc | Mopeds, small scooters, some pit bikes |
| 125 cc | Learner motorcycles, commuter scooters |
| 250 cc | Dual-sport, entry sportbike, small ATV |
| 450 cc | Motocross, mid-size ATV |
| 650 cc | Middleweight twins, big thumpers |
| 1000 cc | Litre-class sportbikes |
Why your figure often reads just under the badge
Manufacturers build right up to a class limit and then round the badge upward. A "250" is frequently 249 cc, a "600" supersport is often 599 cc, and a "450" motocrosser might be 449 cc. That is not an error in your measurement — it is the badge rounding to a clean class number the real geometry never quite reaches.
Reading cc across to cubic inches
Even on a metric build you sometimes need the imperial figure — a piston spec, an American forum thread, a class written in cubic inches. The panel shows the cubic-inch equivalent beside every cc result using the exact 16.387064 factor, so a 1,600 cc four reads straight across to about 97.6 cu in without a second tool.
Precision when it matters
Because bore is squared, a small bore error moves the result more than a small stroke error. For a rough class check, measuring to a tenth of a millimetre is fine; for a build, use a bore gauge to 0.01 mm and switch the panel to Build precision to see the extra decimals. A worn or rebored cylinder is fractionally larger than nominal, so measure the actual bore rather than trusting the badge once an engine has been machined.
References & standards
- NIST SP 811 — Guide for the use of the SI, including the exact inch and unit-conversion factors.
Frequently asked questions
How do I calculate cc from bore and stroke in mm?
Take π/4 × bore² × stroke with both in millimetres to get mm³ per cylinder, divide by 1000 for cc, then multiply by the cylinder count. This page does all of that from your two measurements.
Why divide by 1000 when working in millimetres?
Because π/4 × bore² × stroke in mm gives cubic millimetres, and there are exactly 1000 mm³ in one cc. Forgetting that step is the most common metric mistake.
What is a 250 cc single in bore and stroke terms?
A common 250 single is around 76 mm bore × 55 mm stroke, which works out to about 249.5 cc — the geometry behind the '250' badge.
How do I get total cc for a multi-cylinder bike?
Work out one cylinder's cc, then multiply by the number of cylinders. A 600 cc inline-four is roughly 150 cc per cylinder across four pots.
Do I enter bore or radius in the mm calculator?
Bore — the full cylinder diameter. The π/4 term already accounts for the radius internally, so measuring across the bore is correct.
Can this convert my cc result to cubic inches?
Yes. Every cc figure is shown alongside its cubic-inch equivalent using the exact 16.387064 factor, so a metric build reads straight across to CID if you need it.
What cc class does my engine fall into?
The panel names the class — 125, 250, 450, 650 or litre — as soon as it has a total, so a metric result immediately tells you the licensing or racing bracket.
My bore is in mm but stroke is in inches — now what?
Convert one so both share a unit before calculating. Switching the tool's unit toggle converts the number you already typed rather than just relabelling it.
Is 1000 cc the same as 1 litre?
Yes, exactly. So a 998 cc twin is 'about a litre', and a 1600 cc four is 1.6 L. The litre figure is shown next to your cc result automatically.
How precise is a cc figure from measured bore?
As precise as the measurement. A bore gauge reading to 0.01 mm gives a cc figure good to a fraction of a percent; a rough caliper check is only an estimate.
Why is my calculated cc a little under the badge?
Marketed cc figures are rounded up to a round class number. A '250' may be 249 cc by geometry, and a '600' supersport is often 599 cc.
Can I use this for a two-stroke?
Yes for the swept volume, which is identical geometry. Just remember two-strokes fire every revolution, so their power character differs even at the same cc.
Does bore wear change the cc?
Slightly. A worn or reground bore is fractionally larger, adding a little cc; measure the actual bore if the engine has been machined.
How many cylinders can I enter in mm mode?
One through sixteen. Enter the real count — the tool multiplies per-cylinder cc by it to give the honest total for singles up to big multis.
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