Rule 28 Performance Lab
Rule 28 built four calculators for the Performance Lab: Power Profiler, Aero Calculator, Nutrition Calculator, and Rolling Resistance Calculator. All four are free, all four run on your own numbers rather than population averages, and all four are built on the same physiology and exercise-science models used in sports labs — just without the sports lab. This post walks through what each one does, the research behind it, and how they connect.
The four tools, at a glance
| Calculator | What it tells you | What you need |
|---|---|---|
| Power Profiler | Your critical power, W′, and which of five energy systems is holding your discipline back | Power numbers across a few durations |
| Aero Calculator | Your CdA (drag) in two positions or kit setups, and what the gap costs in time and watts | Two field tests: power, speed, weight |
| Rolling Resistance Calculator | Your Crr (rolling resistance) for two tyre, pressure, or surface setups | Two field tests: power, speed, weight |
| Nutrition Calculator | Carbohydrate, fluid, sodium and caffeine targets for a specific race or ride | Duration, power, weight, conditions |
Power Profiler: find out what's actually holding you back
Power Profiler takes your best power output across a handful of durations — 5 seconds through to an hour — and fits a critical power curve to it. Critical power (CP) and W′ are not new ideas: they come from work by Monod and Scherrer in 1965, who first described the hyperbolic relationship between how hard you work and how long you can sustain it, later extended to cycling by Moritani and colleagues. CP is roughly the intensity you could hold in a steady state for a long time; W′ is the finite reserve of work you can spend above it before you're forced to slow down.
From that curve, Power Profiler scores five energy systems — neuromuscular power, anaerobic capacity, aerobic power, threshold, and durability — against what your specific discipline actually demands. A time triallist and a criterium racer with identical numbers get different verdicts, because a TT lives and dies on threshold and a crit lives and dies on repeated short efforts. Whichever system is weakest relative to your discipline becomes your limiter, and the tool hands you a session, in your own watts, built to train it.
Aero Calculator: find your real CdA without a wind tunnel
Aerodynamic drag is the single biggest force working against you above about 25 km/h, and the honest way to measure it — a wind tunnel — is out of reach for almost everyone. The alternative most serious time triallists and triathletes actually use is field testing: the Chung method, developed by Robert Chung, which uses nothing more than a power meter and a speed sensor to back-calculate CdA from real riding.
Our Aero Calculator uses a simplified two-test version of that method. Ride the same stretch of road twice — once in each position or kit setup you're comparing — enter the power, speed and weight from each, and it calculates the CdA for both, then translates the gap into the time and power it's actually worth over a distance you choose. If you want the fuller version of this — testing more than two setups in one file, using Golden Cheetah's virtual elevation matching — we've also published a step-by-step guide to aero testing at home.
Rolling Resistance Calculator: the companion test
Rolling Resistance Calculator is the same method aimed at the other major force acting against you on the flat. It holds your CdA constant and solves for Crr instead — so where the Aero Calculator isolates position and kit, this one isolates tyres, pressure, and surface. Test two tyre choices in the same position on the same road and it'll tell you which one is actually faster, not which one feels faster, plus the same time-and-power translation.
The two tools are designed to feed each other. If you've already run the Aero Calculator, take your measured CdA and drop it into the Rolling Resistance Calculator's advanced settings instead of the default assumption — you'll get a tighter, more personal number back.
Try the Rolling Resistance Calculator →
Nutrition Calculator: fuel the effort you're actually doing
Most online fuelling calculators give you one number regardless of how long you're riding. Ours doesn't, because the science doesn't support a flat number — how much carbohydrate you can use scales with duration, not just intensity. A 45-minute race and a 4-hour gran fondo at the same relative effort don't call for the same intake, and current guidance from bodies including the American College of Sports Medicine and the International Society of Sports Nutrition (ISSN) reflects that with duration-banded targets rather than a single figure.
Above roughly 60g of carbohydrate an hour, a single sugar source stops being the limiting factor — your gut is. Research from Asker Jeukendrup and others on "multiple transportable carbohydrates" showed that combining glucose and fructose, which use separate absorption pathways, can push usable intake to 90–120g/hour. The calculator flags this automatically once your target crosses that line.
Fluid is handled differently to most tools too. Rather than a fixed number, it returns a range with an explicit instruction to drink to thirst within it — that's a deliberate reflection of the 2015 International Exercise-Associated Hyponatremia Consensus Conference, which found that forcing down a fixed volume regardless of thirst doesn't help performance and can be genuinely dangerous. Sodium is modelled in the style of the sweat-sodium research published by Alan McCubbin, and the calculator adds an evidence-based caffeine dose — around 3mg/kg, per the ISSN's position stand — which the original version of this tool didn't cover at all.
Try the Nutrition Calculator →
How the four fit together
Used on their own, each tool answers a narrow question. Used together, they build a fuller picture of a specific race:
- Run the Aero Calculator and Rolling Resistance Calculator to get your real CdA and Crr, rather than assumed averages.
- Run Power Profiler to get your CP and W′, and to know which energy system to train between now and race day.
- With CP, CdA and Crr all your own, you have a genuinely personal estimate of what speed you can hold — which sets your expected duration.
- Feed that duration, your power, and the day's conditions into the Nutrition Calculator for a fuelling plan matched to the effort you're actually about to do.
None of the four require each other to be useful on their own — but the numbers do genuinely improve when you use them as a set.
Frequently asked questions
What is critical power in cycling?
Critical power (CP) is roughly the highest power output you can sustain in a metabolic steady state for a long period — commonly estimated from efforts in the 5–20 minute range. It's paired with W′, a finite store of work you can do above CP before you're forced to slow down. The concept was first described by Monod and Scherrer in 1965 and is now standard in power-meter-based training.
What is CdA in cycling?
CdA (drag coefficient × frontal area) is a single number describing how aerodynamic a rider and bike are — lower is faster for the same power. It's typically measured in a wind tunnel or estimated in the field using power, speed and elevation data via the Chung method.
How much carbohydrate should I eat per hour while cycling?
It depends mainly on duration: current guidance runs from close to nothing under 45 minutes, up to roughly 60g/hour for rides in the 1.5–2.5 hour range, and up to 90–120g/hour for efforts beyond 2.5–3 hours — provided you use a glucose:fructose mix above 60g/hour, since a single carbohydrate source is limited by gut absorption around that point.
Should I drink a fixed amount of fluid per hour on the bike?
No — current sports-medicine consensus explicitly advises against a fixed fluid prescription. The 2015 International Exercise-Associated Hyponatremia Consensus Conference recommends drinking to thirst within a sensible range based on your sweat rate, rather than forcing down a set volume regardless of how you feel.
What's a good Crr for road bike tyres?
Well-made road tyres at reasonable pressure typically sit around 0.003–0.006 Crr on smooth tarmac; gravel and off-road tyres run higher. Small differences matter — moving from 0.008 to 0.004 roughly halves your rolling resistance power at a given speed.
Are the Performance Lab calculators free?
Yes. All four are free to use on rule28.com, with no account or purchase required.
