How our equipment and performance calculators work
These tools do not size a frame. They implement published models for gearing, wheels, tire pressure, power, and climbing rate. Some of them are exact arithmetic. Others are estimates with real error bars, and the difference matters, so this page says which is which.
Drivetrain and wheel math
Everything in this group is deterministic. Given the same teeth, chainstay, or rim numbers, you get the same answer we do, and you can check it by hand.
| Calculator | What it computes | Assumptions and limits |
|---|---|---|
| Gear ratio | Ratio = chainring teeth divided by cog teeth. Gear inches = ratio x wheel diameter in inches. Development = ratio x wheel circumference in metres. | Pure tooth counting, so it is exact. It says nothing about whether a chain will physically reach a given combination on your frame. |
| Speed and cadence | Speed in km/h = ratio x wheel circumference in metres x cadence x 60 / 1000. The same relation solved the other way gives the cadence needed for a target speed. | Assumes no wheel slip and no freewheeling. Real average speed on a ride is lower because you coast, brake, and stop. |
| Skid patch | Single-leg patches = cog teeth divided by the greatest common divisor of chainring and cog. If that number is odd, an ambidextrous rider doubles it. | A fixed-gear result and a whole-number one. It counts distinct contact points, not how quickly you will wear the tire through. |
| Chain length | The Shimano method: links = 2 x chainstay in inches + largest chainring / 4 + largest cog / 4 + 1, rounded up to the next even number. | A starting length for a conventional derailleur drivetrain. Suspension frames, chain guides, and long-cage clutch derailleurs can need more. |
| Spoke length | The standard wheelbuilding triangle: the square root of rim radius squared plus flange radius squared plus hub offset squared, minus twice rim radius x flange radius x the cosine of the lacing angle. | Only as good as your ERD, flange, and offset numbers. It rejects odd or under-12 spoke counts and cross patterns a wheel cannot physically be laced to. |
- Gear ratio
- What it computes
- Ratio = chainring teeth divided by cog teeth. Gear inches = ratio x wheel diameter in inches. Development = ratio x wheel circumference in metres.
- Assumptions and limits
- Pure tooth counting, so it is exact. It says nothing about whether a chain will physically reach a given combination on your frame.
- Speed and cadence
- What it computes
- Speed in km/h = ratio x wheel circumference in metres x cadence x 60 / 1000. The same relation solved the other way gives the cadence needed for a target speed.
- Assumptions and limits
- Assumes no wheel slip and no freewheeling. Real average speed on a ride is lower because you coast, brake, and stop.
- Skid patch
- What it computes
- Single-leg patches = cog teeth divided by the greatest common divisor of chainring and cog. If that number is odd, an ambidextrous rider doubles it.
- Assumptions and limits
- A fixed-gear result and a whole-number one. It counts distinct contact points, not how quickly you will wear the tire through.
- Chain length
- What it computes
- The Shimano method: links = 2 x chainstay in inches + largest chainring / 4 + largest cog / 4 + 1, rounded up to the next even number.
- Assumptions and limits
- A starting length for a conventional derailleur drivetrain. Suspension frames, chain guides, and long-cage clutch derailleurs can need more.
- Spoke length
- What it computes
- The standard wheelbuilding triangle: the square root of rim radius squared plus flange radius squared plus hub offset squared, minus twice rim radius x flange radius x the cosine of the lacing angle.
- Assumptions and limits
- Only as good as your ERD, flange, and offset numbers. It rejects odd or under-12 spoke counts and cross patterns a wheel cannot physically be laced to.
Tire pressure
Tire pressure is the one place we use a shaped model rather than plain arithmetic. It follows the Berto approach of pressure per unit of load per unit of tire width. Rider and bike mass are added, split front and rear by a weight fraction that depends on terrain, and turned into a starting pressure. Terrain then scales the result down, from no change on smooth road to a 25 percent reduction for mountain use, and a tubeless setup takes a further 8 percent off.
The output is clamped to a sane band for the tire width you entered, and shown as a range of roughly plus or minus 3 psi. Treat it as a place to start on the first ride, not a setting. Rim width, casing, insert use, and how you like the bike to feel all move the right answer, and none of them are inputs here. The pressure printed on the tire sidewall and the limits published by your rim manufacturer override anything this tool says.
Power, calories, and climbing
This group is the least certain on the site. The physics is sound and the constants are conventional, but the inputs are estimates about your body and your position, so the outputs inherit that uncertainty.
| Calculator | What it computes | Assumptions and limits |
|---|---|---|
| Power estimator | The standard four-term road model: gravity, rolling resistance, aerodynamic drag, and drivetrain loss, summed at a given speed. Gravity uses 9.8067 m/s squared and drag uses air density 1.225 kg/m cubed. | Air density is fixed at sea level and 15C, so mountain-pass numbers run high. CdA and rolling-resistance values come from a preset list, not from measuring you. |
| FTP and watts per kilo | Watts divided by body mass, then bucketed: under 2.0 untrained, under 3.0 recreational, under 4.0 trained, under 5.0 competitive, and above that elite or pro. | The bands are a rough orientation, not a test protocol. They do not account for age, sex, or the duration your FTP was measured over. |
| Calorie burn | From power: kcal = watts x seconds / (efficiency x 4184), with cycling efficiency of 24 percent. From effort level: kcal = MET value x body mass in kg x hours, using the published MET table from 4.0 for leisure to 12.0 for racing. | Both are population averages. Individual gross efficiency varies, and a MET band covers a wide spread of real efforts. |
| Climbing VAM | Vertical metres gained divided by hours climbing. Gradient is elevation gain over horizontal distance as a percentage. Results are bucketed from recreational under 600 to professional at 1500 and above. | VAM is only comparable between climbs of similar gradient. On a shallow climb the same rider posts a much lower number. |
- Power estimator
- What it computes
- The standard four-term road model: gravity, rolling resistance, aerodynamic drag, and drivetrain loss, summed at a given speed. Gravity uses 9.8067 m/s squared and drag uses air density 1.225 kg/m cubed.
- Assumptions and limits
- Air density is fixed at sea level and 15C, so mountain-pass numbers run high. CdA and rolling-resistance values come from a preset list, not from measuring you.
- FTP and watts per kilo
- What it computes
- Watts divided by body mass, then bucketed: under 2.0 untrained, under 3.0 recreational, under 4.0 trained, under 5.0 competitive, and above that elite or pro.
- Assumptions and limits
- The bands are a rough orientation, not a test protocol. They do not account for age, sex, or the duration your FTP was measured over.
- Calorie burn
- What it computes
- From power: kcal = watts x seconds / (efficiency x 4184), with cycling efficiency of 24 percent. From effort level: kcal = MET value x body mass in kg x hours, using the published MET table from 4.0 for leisure to 12.0 for racing.
- Assumptions and limits
- Both are population averages. Individual gross efficiency varies, and a MET band covers a wide spread of real efforts.
- Climbing VAM
- What it computes
- Vertical metres gained divided by hours climbing. Gradient is elevation gain over horizontal distance as a percentage. Results are bucketed from recreational under 600 to professional at 1500 and above.
- Assumptions and limits
- VAM is only comparable between climbs of similar gradient. On a shallow climb the same rider posts a much lower number.
If you own a power meter, believe the power meter. These estimates are for the case where you do not.
Where the numbers come from
Wheel and tire dimensions, groupset tooth counts, and the preset lists for drag and rolling resistance are stored as data rather than typed into each calculator, so one correction fixes every tool that reads them. How those records are sourced and rechecked is set out in how we source and update data. Inputs are validated before anything is computed: a negative weight, a zero cadence, or an impossible lacing pattern returns an error instead of a confident-looking number.
What we do not do
- We do not test any of this. No pressure here was validated on a rolling road, no CdA was measured in a tunnel, and no calorie figure came from a metabolic cart.
- We do not tune a model to favour a product. Affiliate links pay for the site and are labelled, as the affiliate disclosure explains. They do not touch the arithmetic.
- We do not present an estimate as a measurement. Where a model is approximate, the page says so rather than quietly adding decimal places.
Found an error?
If a result here does not match the model described on this page, or a constant is wrong, report it on the public issue tracker. Reports are handled in the open. We aim to respond within five working days, and a confirmed error gets fixed in the shared math module, which corrects every calculator that uses it.
This page is rechecked against the calculator source whenever a model changes, and at minimum once every twelve months, on the review cycle described in the editorial policy.