[CALCULATOR EMBEDDED HERE. Build spec below.]
What the calculator is checking
Three limits, and a bike has to clear all three.
Maximum power, 35 kW. Roughly 47 bhp. The one everybody knows.
Power to weight, 0.2 kW/kg. Divide power by kerb weight. This is the one that catches light bikes, because a 150 kg machine hits the ratio limit at 30 kW and never gets near the 35 kW cap.
Unrestricted power, 70 kW. What the machine produced before anybody restricted it. A bike must not be derived from a machine making more than twice the A2 power, so a heavily-restricted litre bike does not count, but a naturally A2-spec bike does.
Getting the numbers right
The calculator is only as good as what you put in, and two of the three inputs are commonly wrong.
Kerb weight, not dry weight. Manufacturers quote dry weights that exclude fuel, oil, coolant and sometimes the battery. A dry figure can be 15 or 20 kg under the real one, which flatters the ratio and gives you an answer that is wrong in the direction that gets you in trouble. Use the kerb or wet weight, and check it against the V5C where you can.
Power before restriction, not after. For a factory A2 machine these are the same number and you can put the same figure in both fields. For a restricted bike they are different, and the second one is the figure that decides whether any restriction can work at all.
Watch the unit. Manufacturer specifications frequently quote PS rather than bhp, and they are not the same. A bike listed at 47 PS is about 46 bhp and comfortably legal. One listed at 47 bhp sits right on the limit.
Where to find the original figure. The manufacturer's specification for that model and variant, in that model year. Power outputs change between generations of the same bike, so a figure from a five-year-old road test may not describe the machine in front of you.
What the answer means
All three pass. The machine sits inside A2 as specified. Confirm the figures against the V5C and the manufacturer's data before you buy.
Power fails. Restriction may bring it into range, provided the unrestricted figure is 70 kW or under.
Ratio fails. A light bike making too much for its weight. Restricting the power fixes it, since reducing power reduces the ratio.
Derivation fails. Nothing fixes it. A machine that made more than 70 kW originally cannot be brought into A2 by any restriction, however well done and however genuine the paperwork. This is the one to check before you travel to see a bike.
Why this catches people out
The restriction certificate is the problem. It is genuine, it is issued by someone competent, and it says the bike now makes 35 kW. All of that can be true while the bike is still outside the category, because the rules ask a second question the certificate does not answer.
A dealer telling you a bike is restricted and therefore fine may be entirely sincere. The certificate proves the first limit and says nothing about the third.
Riding a machine outside your entitlement generally means riding without valid insurance, which turns a licensing technicality into a considerably larger problem.
FAQs
What is the A2 power limit in bhp?
35 kW is approximately 47 bhp. The legal figure is in kilowatts, so work in kW and treat bhp as the conversion rather than the other way round.
Can any bike be restricted to A2?
No. If it produced more than 70 kW before restriction, it is outside the category permanently.
Does the power-to-weight limit use dry weight?
No. Kerb weight, meaning the machine ready to ride with fluids. Using a dry weight will give you an answer that looks better than the truth.
Is a 500cc bike automatically A2 legal?
Capacity is not one of the three tests. Some 500s comply and some do not, and some considerably larger machines do. Run the numbers rather than assuming from engine size.