Aero calculator
Say what you ride now on the left and what you are planning on the right, and every change — from how you sit to a disc wheel to a whole time-trial bike — comes back in the only two numbers that mean anything here: the watts it saves you, and what that is worth in time on the day.
On the bike above, 250 W over 40 km is 1:08:41, at 34.9 km/h.
These are rough estimates, and they are meant to point you in a direction rather than give you a number. Drag areas differ from rider to rider by more than most of the changes below are worth, and the only way to know what a helmet or a skinsuit does for you is to test it — back to back, on the same road, on the same day. Take the order seriously and the decimals not at all.
Where those watts are going
Air 79%Rolling 21%
The bigger the air's share, the more aero changes are worth. Raise the slope and watch weight take over.
The same parts in the same order as the panel on the left, so the thing you just changed is in the same place on both sides. Inside each part the options climb from what you have towards the most aero, and one choice per part — picking a second option replaces the first, because that is what a bike is.
| Change | Watts saved | Time over 40 km |
|---|---|---|
| Positionup to 67.3 W | ||
| — | — | |
| 14.1 W | 1:32 | |
| 25.3 W | 2:49 | |
| 39.1 W | 4:29 | |
| 67.3 W | 8:12 | |
| Frameup to 5.4 W | ||
| — | — | |
| 5.4 W | 35 s | |
| Handlebarsup to 2.2 W | ||
| — | — | |
| 2.2 W | 14 s | |
| Helmetup to 8.4 W | ||
| — | — | |
| 2.2 W | 14 s | |
| 3.9 W | 25 s | |
| 8.4 W | 55 s | |
| Clothingup to 7.3 W | ||
| — | — | |
| 4.5 W | 29 s | |
| 7.3 W | 47 s | |
| Wheelsup to 7.8 W | ||
| — | — | |
| 1.7 W | 11 s | |
| 4.1 W | 27 s | |
| 5.2 W | 34 s | |
| 7.8 W | 51 s | |
| Tyresup to 18.1 W | ||
| — | — | |
| 8.2 W | 52 s | |
| 14.0 W | 88 s | |
| 18.1 W | 1:53 | |
| Weightup to 1.3 W | ||
| — | — | |
| 0.3 W | 2 s | |
| 1.3 W | 8 s | |
Watts are saved at the same speed; time is saved at the same watts. Drag and rolling figures are typical values from published wind-tunnel, field and rolling-resistance tests, so your own gain can be half or double. The order of the list is more reliable than any one number in it. Prices are not here on purpose: a figure this page invented for a whole bike would be wrong the day it was written, and wrong in a column headed with a currency reads as a measurement. What each change costs is between you and a shop. Still air, sea-level density.
Less than almost anyone expects. Holding 250 W over 40 km, 45 mm carbon wheels in place of box-section alloy are worth about 4 W and half a minute; a rear disc with a deep front, about 8 W. Both are real, and both are near the bottom of the list on this page. Riding on the hoods with your forearms flat is worth about 25 W over the same effort, and costs nothing.
Position first, tyres second. Getting lower and narrower — the drops instead of the hoods, forearms flat — is worth more watts than any component on this page and is free. After that it is tyres, not wheels: on the same 250 W effort, fast tyres with latex or tubeless are worth about 18 W against durable training tyres, which is more than a disc wheel, an aero helmet and a skinsuit put together.
Because a price this page invented would be wrong the day it was written, and wrong in a column headed with a currency reads as a measurement. Watts and seconds are things the physics can stand behind. What a change costs is between you and a shop.
Treat the order as reliable and the decimals as not. The drag and rolling figures are typical values from published wind-tunnel, field and rolling-resistance tests, and riders differ from each other by more than most of the changes here are worth. The only way to know what a helmet does for you is to test it back to back, on the same road, on the same day.
Sooner than most people think. The power it takes to push through the air grows with the cube of your speed, so it dominates a fast flat ride and fades away on a steep climb — but even at 30 km/h on the flat, air is already about three-quarters of what a typical rider is pushing against. The split at the top of the results says which case you are in.