FTP vs Critical Power: How to Calculate Your True Cycling Zones

The debate in cycling training forums looks like an advanced physiology seminar. They argue FTP against Critical Power (CP). They compare Functional Reserve Capacity (FRC) to W'. While academics debate which mathematical model best represents human fatigue in a spreadsheet, you have a much more grounded problem.

You need a real number to set your training zones on your bike computer and know what power to ride at tomorrow morning.

For an amateur cyclist training 6 to 10 hours a week, the problem is rarely the statistical model your software chooses. The problem is that the input data—the test you did to calculate your threshold—is usually contaminated from the first minute. A bad test does not give you a suboptimal number. It gives you a false number that will ruin your workouts for the next three months.

This article explains why traditional tests fail, the real difference between FTP and Critical Power, and how to execute a field test that yields usable zones.

The 0.95 Lie in FTP

FTP (Functional Threshold Power) is classically defined as the maximum power you can sustain steadily for 60 minutes. It is your anaerobic threshold expressed in watts.

The practical problem is obvious. Nobody wants to do a maximum 60-minute test. It is mentally exhausting, interferes with your training week, and finding a flat road without traffic lights for a full hour is impossible for most.

The industry's solution was the 20-minute test. You ride all-out for 20 minutes, take the average power, and multiply it by 0.95. That 5% discount assumes your power would drop by that proportion if you had to continue for 40 more minutes.

This is where the system breaks. That 0.95 is a population average. If you come from explosive sports, lift heavy weights, or simply have fast-twitch dominant genetics, your anaerobic capacity is high. In a 20-minute effort, that anaerobic battery provides a massive amount of energy. Your power drop by minute 60 would not be 5%, but 10% or 12%.

If you average 300w for 20 minutes, the formula gives you an FTP of 285w. But your actual FTP might be 265w. The following Tuesday, when your plan prescribes 4 sets of 10 minutes at 285w, you will not get past the second rep. You will feel like you failed, when in reality you were trying to train in your VO2 Max zone believing it was your threshold zone.

The ramp test (Zwift) is even worse

The ramp test increases resistance every minute until you cannot turn the pedals. Your FTP is calculated by taking 75% of your best minute. It is convenient because it only hurts for the last three minutes and requires zero pacing strategy.

But the ramp test measures your maximum aerobic power (VO2 Max) and your lactate tolerance, not your steady state. For cyclists with a poor aerobic engine but good muscular strength, the ramp test systematically overestimates FTP. You end up with an inflated number to show off, but a useless number for training.

Critical Power (CP): Better Math, Worse Execution

Critical Power solves the 0.95 problem. Instead of guessing how your power drops, it asks you to do two or three max efforts of different durations (for example, a 3-minute test and a 12-minute test on different days).

With that data, the software plots a curve and separates your physiology into two numbers:

• CP (Critical Power): The aerobic baseline you can theoretically sustain without accelerated metabolic fatigue. This is your true threshold.
• W' (W prime): Your anaerobic battery, measured in kilojoules. The extra energy you can spend above your CP before you collapse.

On paper, Critical Power is superior. In real life, it has a fatal flaw for the amateur athlete: execution.

For the mathematical model to work, the 3-minute and 12-minute tests must be absolute, perfectly paced maximal efforts. Asking an amateur cyclist to do a 3-minute test with perfect pacing guarantees a disaster. They sprint out at 500w, their legs flood with lactic acid in 45 seconds, and they end up crawling at 200w. That erratic data ruins the mathematical curve and, once again, leaves you with incorrect zones.

The Solution: Strict Execution Protocol

The difference between training with FTP or CP is marginal for 99% of cyclists. What actually changes your progression is testing consistently, under the same conditions, and depleting your anaerobic battery before measuring your aerobic threshold.

If you are going to use the 20-minute test, the classic Allen and Coggan protocol demands a 5-minute all-out effort before the main test. That preliminary effort is not a warm-up. It is a depletion effort. It serves to drain your anaerobic battery (your W') so that when you start the 20 minutes, you are forced to generate power almost exclusively with your aerobic system. Only then does the 0.95 multiplier become accurate.

The Three Non-Negotiable Rules of Field Testing

A power number is only valid if the context in which it was obtained is reproducible. If you break these rules, invalidate the test and repeat it the following week.

• Environment specificity: Your indoor FTP is usually 10w to 20w lower than your open road FTP, due to cooling, inertia, and bike rigidity. If you are doing an indoor training block, test on the trainer. If you are riding outside, test on a steady 3% to 5% climb. Never cross the data.
• No ERG mode: The 20-minute test is done using your bike's gears and finding your own cadence. The trainer's ERG mode forces you to hold a fixed power dictated by the machine, which eliminates your ability to micro-manage fatigue. Turn ERG off.
• Standardized rest: You cannot test after a heavy training week, nor after three days of complete inactivity (your legs will be blocked). The test is done after two days of light training, with full glycogen stores.

How to Use the Number: Training Zones

Once you have an accurate threshold, we set your zones. At Triaperformance, we use a power-based model. All our pure cycling plans require a power meter, because it is the only metric that measures actual mechanical work without the lag of heart rate.

There are three critical zones derived from your test. The rest are transitions.

Zone 2 (Aerobic Endurance): 56% to 75% of threshold.
Here you build mitochondrial density. This is where you should spend 80% of your saddle time. The most common error is riding at 80% of FTP believing it is Zone 2. That upper bracket generates residual fatigue without providing extra aerobic benefits.

Sweet Spot: 88% to 93% of threshold.
Just below threshold. It generates adaptations very similar to training at 100% of your FTP, but requires much less recovery time. It is the key zone for build blocks.

Zone 4 (Threshold): 95% to 105% of threshold.
The specific work to push your FTP up. Intervals here hurt, demand deep concentration, and deplete your glycogen rapidly. If your original test overestimated your FTP, this zone will be physically impossible to sustain for more than 15 minutes total.

To apply these numbers in a logical structure that guarantees progression, the choice of loading and deloading weeks is critical. These are the frameworks we use to turn that number into real performance:

How Often to Re-Test

FTP is not a static metric, nor does your ego depend on it going up every week. Physiological adaptations take time. Testing too frequently interrupts training and generates unnecessary fatigue.

The optimal cycle is every 8 to 12 weeks, at the start of a new mesocycle. However, there are two field indicators that tell you your FTP has become outdated prematurely:

• Your heart rate drops during threshold efforts: If you are doing intervals at 100% of your prescribed FTP, and your pulse stabilizes 10 beats below normal with a moderate perceived exertion, your true threshold has already gone up.
• Sweet Spot feels like Zone 2: 20-minute intervals at 90% should require concentration. If you can hold a fluent conversation at that power, your zones are outdated. Manually bump your FTP up by 5 to 10 watts and evaluate the next session.

The Lab Argument

The technical critique of any field test is valid. Measuring power on the street does not tell you what is happening inside your body. Professional teams do not use 20-minute tests. They use metabolic profiles, spirometry to measure oxygen consumption, and lactate pricks to find the exact inflection points (LT1 and LT2).

It is undeniable that the lab is more precise. If you have the budget and the access, a lactate profile gives you the perfect anchor for your Zone 2.

But the lab has a blind spot: it is a static snapshot of a single day. Your lactate curve changes depending on whether you ate carbohydrates or fats for breakfast, the accumulated muscle fatigue that week, and general stress. A field test with a power meter is free, highly specific to the environment where you will compete, and exactly repeatable every two months.

To build long-term fitness, the repeatability of a well-executed field test will always beat the precision of a lab test you only do once a year. Pick a protocol, respect the preliminary depletion effort, calculate your zones, and focus on executing the plan.