How much time does a kilogram save on a climb?
About one percent of the climb, on a steep one — which is a lot less than the price tags suggest and a lot more than nothing. Climbing is the one place weight really is the lever: on an 8% grade about 90% of your power is lifting mass against gravity, so shedding a share of the mass buys nearly the same share of speed. For a 75 kg rider on an 8.5 kg bike holding 250 W up Alpe d'Huez (13.8 km at 8.1%), one kilogram removed — from anywhere — turns a 67 min 54 s climb into a 67 min 10 s one: 44 seconds, 1.1% of the climb, or the same as finding 2.9 W. The calculator runs that for your climb, your power and your kilograms.
| Removed | Box Hill (UK / Zwift) 2.5 km · 5% · 8 min 26 s | Old La Honda (CA) 5.4 km · 7.3% · 24 min 20 s | Alpe d'Huez 13.8 km · 8.1% · 67 min 54 s |
|---|---|---|---|
| 0.5 kg (1.1 lb) | 2 s | 8 s | 22 s |
| 1 kg (2.2 lb) | 5 s | 15 s | 44 s |
| 2 kg (4.4 lb) | 9 s | 31 s | 1 min 29 s |
| 3 kg (6.6 lb) | 14 s | 46 s | 2 min 13 s |
| 5 kg (11 lb) | 23 s | 1 min 17 s | 3 min 41 s |
83.5 kg rider + bike at 250 W, still air, road tires — the same Martin power model as the climb time calculator, so the two pages can't disagree. Savings scale almost linearly with kilograms and with climb length.
Bike weight or body weight — does it matter which?
Not to the hill. Gravity pulls on the total, and at steady climbing speed a gram on the rim, the frame, the bottle cage or the rider costs exactly the same to lift — the "rotating weight counts double" story is true only while accelerating, and the wheel-weight page prices that at a couple of seconds per hundred surges. What differs is where the kilograms are available. A modern road bike has perhaps one to two kilograms of removable weight in it, and the last of them cost thousands. Most riders carry several removable kilograms themselves, and a full bottle is three quarters of one.
A full 750 ml bottle weighs 750 g. Left at the bottom — or drunk in the first ten minutes, which is what the pros do before a summit finish — it's worth 33 s on Alpe d'Huez at 250 W. A 400 g lighter wheelset is worth 18 s for around $2,400. The second bottle is the upgrade nobody sells.
What is a kilogram worth in watts?
Riders think in watts, so here's the kilogram converted: the extra power the original weight would need to match the lighter rider's time, at each grade. It's the number that settles most "should I buy it or train" conversations:
| Grade | Speed at 250 W | 1 kg is worth | Share of 250 W |
|---|---|---|---|
| Flat | 36.6 km/h | 0.5 W | 0.2% |
| 2% | 27.7 km/h | 1.9 W | 0.8% |
| 4% | 20.5 km/h | 2.6 W | 1% |
| 6% | 15.6 km/h | 2.8 W | 1.1% |
| 8% | 12.3 km/h | 2.9 W | 1.2% |
| 10% | 10.1 km/h | 3 W | 1.2% |
| 12% | 8.6 km/h | 3 W | 1.2% |
83.5 kg system, still air. On a steep grade a kilogram is close to its share of the mass — 1.2% of your power — because nearly all the power is lifting; on the flat it's under a watt of rolling resistance. A month of training at threshold adds more than 2.9 W for most riders, for free.
Weight or aero — which matters on your roads?
The two levers trade places with the grade. Over the same 10 km at 250 W, one kilogram removed against one hundredth of a square meter of drag area removed (a good helmet, a set of deep wheels, or tucking your elbows):
| Grade | −1 kg saves | −0.01 m² CdA saves | Which wins |
|---|---|---|---|
| Flat | 1 s | 10 s | Aero |
| 2% | 6 s | 9 s | Aero |
| 3% | 9 s | 8 s | Weight |
| 4% | 14 s | 6 s | Weight |
| 5% | 18 s | 5 s | Weight |
| 6% | 23 s | 4 s | Weight |
| 8% | 32 s | 3 s | Weight |
| 10% | 40 s | 2 s | Weight |
10 km at each grade, 83.5 kg, CdA 0.32 → 0.31, still air. The kilogram overtakes the aero change at about 3%; below that, the aero savings calculator is the page to argue with. Most rides, and every flat time trial, live on the aero side of that line; a summit finish lives on this one.
Dollars per second: the usual upgrades, priced
Every kilogram saves the same seconds. What differs is what it costs to find one. The usual candidates, on Alpe d'Huez at 250 W:
| Change | Grams | Saved on the Alpe | Typical cost | $ per second |
|---|---|---|---|---|
| Leave a full 750 ml bottle behind (or drink it early) | 750 g | 33 s | Free | — |
| Two kilograms off the rider (4.4 lb) | 2000 g | 1 min 29 s | Free | — |
| Lighter tires and tubes (or tubeless) | 150 g | 7 s | $120 | $18 |
| Lighter saddle | 100 g | 4 s | $200 | $45 |
| Carbon handlebar | 120 g | 5 s | $300 | $56 |
| Top-tier cassette and chain | 100 g | 4 s | $350 | $79 |
| Lightweight carbon wheelset | 400 g | 18 s | $2,400 | $135 |
| Lighter frameset | 600 g | 27 s | $3,500 | $131 |
Grams are typical differences between mid-range and top-tier parts; costs are rough US retail for this column only. Two changes add simply here — unlike aero, kilograms don't interact.
On the same climb, at the same power. The frameset is $131 a second and the two kilograms are free — with one condition, covered below. None of this makes light parts pointless: if you've already done the free things, the wheelset's 18 s are real and repeat on every climb you ever ride. It just puts the receipt beside the stopwatch, which the catalog never does.
Does a lighter bike make you faster on the flat?
Barely. On flat ground the only thing a kilogram costs you is rolling resistance, and that's a small term: at 200 W over a flat 40 km, one kilogram less is worth 3 s and about 0.4 W. Rolling hills give some of it back — you lift the kilogram on every rise and recover only part on the descent — but on the flat the honest ranking is tires, then position, then wheels for their shape, then weight a distant fourth. A light bike feels quick because it accelerates eagerly out of every corner; the stopwatch over a flat hour mostly can't tell.
When losing weight makes you slower
The free kilograms have a condition: the power has to come with you. Weight loss that costs muscle or under-fuels training takes watts away faster than it removes kilograms, and the arithmetic turns against you quickly. Our 75 kg rider losing 2 kg cleanly climbs the Alpe 1 min 29 s faster. Lose the same 2 kg but 3% of the power with it (243 W instead of 250) and the climb is 27 s slower than before losing anything — W/kg went from 3.33 to 3.32, and W/kg is what climbs. The fueling calculator prices what rides actually burn; losing weight from the table while eating on the bike is how the kilograms come off without the watts.