* Calculation assumes constant speed and ideal conditions. Drafting savings based on Blocken et al. (2018) — Journal of Wind Engineering.
ρ (rho) = air density in kg/m³. The standard value is ρ 1.225 kg/m³ at sea level at 15°C. At higher temperature, altitude or lower air pressure, ρ decreases — meaning less aerodynamic drag. Adjust this via advanced settings.
Enter two CdA values and see how many watts you save and how much faster you ride.
* Time savings calculated via numerical Newton-Raphson solver. Assumptions: constant speed, no wind, ρ = 1.225 kg/m³.
Wind tunnel verified savings at 40 km/h. Click column header to sort. Check to track budget.
| ✓ | Upgrade | CdA Gain | Saving | Cost | € / Watt |
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⁑ Chain wax and tyres save via rolling/mechanical resistance, not CdA.
†† Internal cables often require a full new frame/cockpit — no fixed price.
Sources: BikeRadar Silverstone wind tunnel (2023), Cyclingnews wind tunnel (2022), Rule 28 / ATP Performance, Blocken et al. (2018).
CdA (Coefficient of Drag × frontal Area, in m²) is the measure of your aerodynamic drag on the bike. The lower your CdA, the fewer watts you need for the same speed. You calculate it by entering your average power, speed, total weight and conditions — this calculator does it for free, instantly online.
A good CdA for an amateur is between 0.280 and 0.330 m² on a standard road bike with hands on the hoods. Fast amateurs on an aero bike reach 0.230–0.280 m², trained time trialists 0.200–0.230 m², and World Tour pros in an optimised TT position get below 0.200 m².
Ride a flat, windless segment at constant speed with a calibrated power meter. Enter your average power, speed and total weight into the calculator. Always ride the segment out and back to average out residual wind, repeat at least three times and use the mean. Early morning is usually the calmest.
W/CdA is your power divided by your CdA — a measure of how fast you are on flat terrain, comparable to what W/kg is for climbing. Two riders with exactly the same power but a different CdA ride at completely different speeds. The higher your W/CdA score, the faster you go on the flat.
Already around 15 to 20 km/h air resistance and rolling resistance are roughly equal. At 30 km/h, air accounts for about 75% of your total resistance, and at 45 km/h nearly 90%. So even on a steady endurance ride at 25 km/h, aerodynamics is by far the biggest factor.
On flat terrain aerodynamics almost always wins. Drag grows with the square of speed, and the power needed to overcome it with the cube. One kilo less makes almost no difference on the flat; only on climbs steeper than roughly 5% does weight really start to count. Rule of thumb: invest in aero for flat rides and time trials, in weight for mountain stages.
The aero position (free, up to ~25W) is by far the best investment. After that, shaved legs (€5, ~7W) and an aero skinsuit (~€150, ~20W) offer the best value per euro. See the full overview with all upgrades in the Aero Gains tab.
An aero helmet saves on average 5–10 watts at 40 km/h compared to a highly vented road helmet, according to wind tunnel tests by BikeRadar and Cyclingnews. The gain depends heavily on head position: a long-tailed aero helmet loses much of its benefit as soon as you drop your head or look around a lot.
A set of deep rims (60 mm+) saves roughly 15 watts at 40 km/h compared to a shallow alloy rim — one of the biggest equipment gains you can buy. But at around €1,000 the price per watt is high. An aero skinsuit, aero helmet and a better position together deliver more for a fraction of the cost. A good investment, but not your first one.
According to wind tunnel and CFD research (Blocken et al., 2018), your drag drops by about 30% directly behind another rider (~10 cm), by ~45% mid-peloton, and up to ~65% at the back of a large, dense peloton. At 40 km/h with a CdA of 0.30 that is roughly 75 to 165 watts.
ρ (rho) is the density of the air in kg/m³ and sits directly in the drag equation: the thinner the air, the less resistance. The standard value is 1.225 kg/m³ at 15°C at sea level. Heat, low air pressure and altitude all lower ρ. At 1,500 m altitude the air is about 14% thinner — at the same power you ride noticeably faster there.
Crr is the rolling resistance coefficient: how much friction your tyres generate on the road. Fast race or track tyres with latex or TPU tubes on smooth tarmac sit around 0.003–0.004, standard race tyres around 0.005, and on rough tarmac or gravel it rises to 0.008 or higher. Rolling resistance is nearly independent of speed, but directly proportional to your total weight.