NichesTools

Gear Ratio & Cadence Calculator

For a derailleur bicycle the distance covered per crank revolution equals gear ratio × wheel circumference. Gear ratio is the number of chainring teeth divided by the number of rear‑cog teeth. If a cyclist wants to know how fast they will go at a given cadence or what gear must be chosen to reach a target speed, we use the simple physics: Speed (km h⁻¹) = Cadence (rpm) × Gear Ratio × Wheel Circumference (mm) × 0.00006. This converts millimetres per minute into kilometres per hour. Converting that to miles uses a standard unit conversion.

Enter the desired riding speed (e.g., 20). The unit is chosen in the next field.
Choose whether the target speed is in kilometers per hour or miles per hour.
Enter the current pedaling cadence in revolutions per minute.
Number of teeth on the front chainring.
Number of teeth on the rear cog or cassette gear.
Total wheel circumference in millimetres. 2100 mm is typical for a 700C road wheel with a 23‑25 c tyre.

What it is

A bicycle's rolling speed is determined by pedal cadence, gear ratio (chainring divided by rear cog or cassette teeth) and wheel circumference. The relationship is governed by a simple kinematic equation that converts millimetres per minute into kilometres per hour. Knowing the target speed—whether for an easy endurance ride or a sprint—it is possible to calculate exactly which gear configuration will give the desired cadence, or conversely what cadence is needed with a given gear. This calculator equips mechanics and riders with precise, real‑world values that can directly inform component selection, drivetrain tuning and training plans.

How to use it

Enter the target speed and select whether it is expressed in km/h or mph. Input your chosen cadence, the number of teeth on the front chainring and rear cog, and the wheel circumference (2100 mm for a standard 700C road bike). The page instantly shows you the equivalent speed in both units, the current gear ratio, the ratio required to hit your target at that cadence, and suggests the nearest practical rear‑cog size.

Worked example

Step 1 – Current gear ratio: 48 teeth ÷ 17 = 2.8235. Step 2 – Convert crank revolutions to travelled distance: 80 rpm × 2.8235 ≈ 225 rev/min of the rear wheel. With a 2100 mm circumference that’s 225 × 2100 = 472,500 mm per minute. Converting mm → km and minutes → hours gives (472 500 ÷ 1 000 000) × 60 ≈ 28.461 kph. Step 3 – Convert to mph: 28.461 kph ÷ 1.609344 ≈ 17.685 mph. Step 4 – Find the ratio needed for 20 kph at 80‑rpm cadence: required = 20 ÷ (80 × 2100 × 0.00006) ≈ 1.984. Step 5 – Recommend a rear cog keeping the front 48 teeth fixed: best integer nearest to 48 ÷ 1.984 ≈ 24 teeth.

Inputs

  • Target Speed: 20
  • Speed Unit: 1
  • Cadence (rpm): 80
  • Chainring Teeth: 48
  • Rear Cog Teeth: 17
  • Wheel Circumference (mm): 2100

Result

  • Speed (kph): 28.461
  • Speed (mph): 17.685
  • Current Gear Ratio: 2.824
  • Gear Ratio Needed for Target Speed: 1.984
  • Suggested Rear Cog: 24

Frequently asked questions

What cadence range is optimal for endurance versus sprint training?

Endurance efforts usually stay between 80–100 rpm; sprinters operate nearer 120‑140 rpm. The calculator shows how a gear change can let you keep within that band while hitting a target speed.

How do tire width or rim size affect wheel circumference and should I adjust for it?

Yes, adding a bigger tubeless tyre or using a larger rim adds roughly 5–15 mm to the circumference. Input your measured circumference to get accurate speeds; manufacturers typically list a 700C‑28c as ≈2100–2120 mm.

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