Power meters changed cycling because watts do not lie about effort. Speed, by contrast, depends on wind, hills, and your body position. This estimator connects the two with physics: given your watts, weight, aerodynamics, and gradient, it estimates your speed, and given a target speed, it tells you the watts that speed demands. That helps you set realistic goals, judge whether a personal best is within reach, and understand why the same effort feels so different on different days.
Three forces resist you on a bike. The first is air resistance, which grows with the cube of speed: doubling your speed needs roughly eight times the power to push the air aside. The second is rolling resistance, which grows steadily with speed and weight, and depends on your tyres and the road surface. The third is gravity on hills, which costs power in direct proportion to gradient and weight. The tool adds all three forces together, which is why its answers match real riding far better than a simple watts-per-kilo rule.
Here is a worked example. An 80-kilo rider and bike, with a typical road drag area of 0.32 and good tyres, holds 200 watts on flat ground. Air resistance takes about 170 watts at 34 kilometres per hour, and rolling resistance takes about 30 watts, giving a total near 200 watts. So 200 watts means roughly 34 kilometres per hour for this rider. The same rider at 300 watts reaches about 40 kilometres per hour, not 51, because air resistance grows so fast. That harsh truth is why aerodynamics matters more than weight on flat roads.
Aerodynamics is the biggest lever most riders ignore. Drag area, written CdA, combines your frontal area with how slippery your shape is. A typical road position is about 0.32, a low aero position can reach 0.25, and an upright position can exceed 0.40. Dropping from 0.32 to 0.25 at the same watts can add 2 to 3 kilometres per hour on the flat, for free. That is why time-trial bikes, aero helmets, and a lower position beat expensive weight savings on flat courses, while weight matters most on steep climbs.
Gradient changes everything. On a 5 percent climb, our 80-kilo rider at 200 watts manages only about 17 kilometres per hour, because gravity now eats most of the power. On a 5 percent descent, the same 200 watts plus gravity pushes the speed past 55 kilometres per hour, and air resistance becomes the limit again. The tool lets you enter any gradient as a decimal, so 0.05 means a 5 percent climb and minus 0.05 means a 5 percent descent. Try your local hill and see why climbing watts per kilo is the number climbers watch.
Weight still matters, but less than many riders think. Rolling resistance and climbing power both scale with total weight, rider plus bike, so losing 5 kilos helps on hills by a few percent. On the flat, weight barely matters, because air resistance dominates. That is why a heavier rider with a good aero position can be faster on the flat than a lighter rider sitting upright. The tool uses rider plus bike weight together, so remember to add the bike, clothing, water, and luggage to your body weight.
The estimator has clear limits. It assumes steady riding with no wind, no drafting, and a constant position. A headwind of 20 kilometres per hour can cost you 3 to 5 kilometres per hour at the same watts, while drafting behind another rider can save 20 to 30 percent of your power. Rough roads, wide tyres, and cold dense air all shift the numbers. Your drag area is also an estimate unless you have tested it in a wind tunnel or with field tests, so treat the result as a guide within a kilometre or two per hour, not an exact promise.
You can use this tool in several practical ways. Before a time trial, enter your sustainable watts and see what speed to expect, then split the course by gradient for a pacing plan. When comparing upgrades, change only the drag area or the weight and see which change buys more speed for your riding. If you train without a power meter, enter the speed you held on a calm flat road and read off the approximate watts, which gives you a training number to work with. In short, it turns watts into speed, and speed back into watts, with honest physics.