Bike Cadence and Speed Calculator (chainring, cog, wheel → km/h at rpm)

Understanding the bike cadence and speed calculator — concept and principle

Your speed on a bike depends on how fast you pedal and which gear you are in. This calculator converts chainring and cog teeth, wheel size, tyre width and cadence into speed in km/h and mph, gear ratio, distance per pedal turn and gear inches, and tells you the cadence needed to hold a target speed.

Each pedal turn moves the bike 6.27 m, so 90 rpm gives 33.8 km/h.

How it is calculated

Wheel outside diameter ≈ bead-seat diameter + 2 × tyre width

Circumference = π × outside diameter

Gear ratio = chainring ÷ cog

Speed (km/h) = cadence × gear ratio × circumference (mm) × 60 ÷ 1,000,000

Development = gear ratio × circumference; gear inches = gear ratio × diameter ÷ 25.4

Cadence for target = target × 1,000,000 ÷ (60 × gear ratio × circumference)

Bead-seat diameters (ISO): 622 mm for 700C/29 in, 584 for 650B, 559 for 26 in, 571 for 650C, 406 for 20 in, 349 for 16 in.

The principle

Each pedal revolution turns the chainring once, pulling the chain past a number of teeth equal to the chainring size. The rear cog then turns chainring ÷ cog times, and the rear wheel with it. Each wheel turn moves the bike forward one circumference. Multiply by pedal revolutions per minute and you have distance per minute, which converts to km/h. Gear inches is an old way to describe the same gear as the diameter of an equivalent penny-farthing wheel.

Worked example

Default inputs: 50-tooth chainring, 17-tooth cog, 700C wheel, 28 mm tyre, 90 rpm, target 35 km/h.

Outside diameter = 622 + 2 × 28 = 678 mm; circumference = π × 678 = 2,130 mm.
Gear ratio = 50 ÷ 17 = 2.941.
Development = 2.941 × 2.130 = 6.265 m per pedal turn; gear inches = 2.941 × 678 ÷ 25.4 = 78.5.
Speed = 90 × 6.265 × 60 ÷ 1,000 = 33.8 km/h = 21 mph.
Cadence for 35 km/h = 35,000,000 ÷ (60 × 2.941 × 2,130) = 93 rpm.

Each point is the calculator re-run with only “Cog teeth (rear)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Cog teeth (rear)	Speed (km/h)
4.25	135.3
8.5	67.7
12.8	45.1
17	33.8
21.2	27.1
25.5	22.6
34	16.9

How to read the result

Many road riders settle at cadences of roughly 80–100 rpm on flat roads, while beginners often pedal slower in bigger gears. If the cadence needed for your target speed is uncomfortably high, shift to a bigger gear; if it is very low, shift down to spare your knees. Comparing gear inches across bikes with different wheel sizes shows which gears are equivalent.

Common mistakes

Using the nominal wheel size (28 in) instead of the ISO bead-seat diameter.
Ignoring tyre width, which changes circumference noticeably.
Expecting measured speed to match exactly; tyres compress under load.
Swapping chainring and cog in the ratio.

Keep in mind: Real tyres sag under load, so actual speed is often 1–2% lower than calculated. Calibrate a bike computer with a measured roll-out for best accuracy.

Frequently asked questions

What is a good cycling cadence?

It varies by rider; many trained road cyclists ride around 80–100 rpm on the flat.

What are gear inches?

Gear ratio times wheel diameter in inches, a traditional way to compare gears.

Why does tyre width matter?

A wider tyre adds height, increasing the wheel's circumference and the distance per turn.

Is 700C the same as 29 inch?

Yes, both use the 622 mm bead-seat diameter; 29-inch tyres are just wider.

Sources: Sheldon Brown, "Tire Sizing Systems" and Glossary "gear inches" (sheldonbrown.com) · ISO 5775 / ETRTO tyre and rim size designations

Checked 9 Nov 2026. ISO 5775 / ETRTO bead-seat diameters (622, 584, 571, 559, 406, 349 mm) and outside radius ≈ rim radius + tyre width: Sheldon Brown, “Tire Sizing Systems” (sheldonbrown.com); gear inches = ratio × wheel diameter (Sheldon Brown, Glossary “gear inches”).

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