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Updated September 09, 2022 | By Bob Fugett

Power to Weight Calculator Explained

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This calculator is the perfect rule of thumb speed per watts calculator.

Before reading below try at least once the free online Power to Weight Calculator which automatically calculates your power to weight, speed per watts, and Race Category based on power. Make up some numbers, doesn't matter, won't hurt a thing.   

Caveat: Only part of the story is told by Watts (whether on the flats or climbing) so be sure to review: Torque.

    1.    How accurate is the calculator?
    2.    Should I include my bicycle weight?
    3.    Can't find formula "t @ P/W" anywhere else on the Internet. What does it mean?
    4.    What is ETA?
    5.    How are watts to mph calculated?
    6.    Why is this calculator so much simpler than all others online?
    7.    Would the watts to mph work in an Excel spreadsheet?
    8.    Why are race categories calculated for men only?

1.   How accurate is the calculator?

From the most accurate to the least accurate (descending order) calculator results are:

  • Power to Weight - dead on absolute accuracy (a matter of simple math)
  • Race Category - statistically correct optimistic reporting based on field data
  • Watts to Speed - in the ballpark though slightly more optimistic than Race Category
  • ETA - the most likely to be in error and still under investigation (11/29/09)


2.   Should I include my bicycle weight?

Consistency is the most important thing, and the best way to get consistently repeatable accurate weight measurements is to use your naked morning weight.

Of course, sharper accuracy can be achieved at any given moment by carefully weighing bicycle, water bottles, clothes, tools, and ancillary components as well, but for tracking your own long term performance those elements have less value if you do not precisely control weight measurement in the exact same way every time.

For comparison with other riders, it is unlikely you will find enough detailed information about their setup to make any extra work on your part meaningful anyway.

If you are a coach/trainer for a team, all equipment and bicycle weight should already have been conformed to a uniform standard, so naked morning weight should work for you as well.

In cases where a few seconds or watts is going to make the difference in who makes the team or not, or who is going to handle what team job during the next race, careful attention to absolute ride weight (as opposed to naked morning weight) is absolutely warranted.

Probably this discussion has been expanded beyond minimal necessity because I like giggling after repeating, "...naked morning weight."

3.   I can't find the formula "t @ P/W" anywhere else on the Internet. What does it mean?

It stands for the amount of time (t) you can hold a specific power (P, in watts) relative to your weight (W, in kilograms).

Stating this concept as the formula t @ P/W  is unique to Cycling Performance Simplified and undoubtedly represents the most refined and useful way to assess your cycling ability ever. 

Track your development based on the amount of time you can hold any given watts, and everything else will fall into place.

The Allen and Coggan table (presented by the Power to Weight Calculator) takes this idea one step further by dividing required efforts into time categories which account for best sprinter, best jumper, best pacer, and best long haul time trialist.

My online calculator does a quick and rough spotting of your Race Categories, but you should compare your own real world results at 5 sec, 1 min, 5 min and FT to the Allen and Coggan table (presented with the calculation) in order to get a baseline on how you stack up to other riders.

Over the widest possible range of riding conditions and courses (discounting team tactics), the rider who holds the highest power to weight ratio for the entire ride or race will always be the winner.

Once you have a handle on your current level of performance based on a simple objective repeatable reference you can work rationally to improve.

This simple formula (t @ P/W) and the concept it embodies is the basis for everything else in Cycling Performance Simplified, so it has its own page: t @ P/W.


4.   What is ETA?

ETA stands for: Estimated Time of Arrival, but in this case it means not to a location but to a terminal velocity.

It is useful to have an understanding of how quickly you can achieve quoted speeds such as 300 watts = 25 mph.

If you start at zero you will not be immediately at 25 mph.

In order to control re-pacing efforts after hard turns and interruptions during time trials, you can use the ETA as a rule of thumb reference for calculating how long at what effort you need to push in order to get back up to pace.

Since your power meter gives a more immediate absolute reference to your actual pace (where current mph speed is always misleading), you can use the ETA number in order to plan your handling of technical difficulties such as extra tight turns and road intersections found on your time trial course.

The formula was taken from an automotive website discussing calculation of 0-60 times then confirmed through review of mechanical physics, motion, and math resources along with empirical real-world testing.

5.   How are watts to mph calculated?

The formula is well known, and Wikipedia provided enough information about dimensionless constants that are typically used in order for SlingShot to get this formula up and running.

Suffice it to say that simplifying rational assumptions were applied to friction and weight variables.

More information can be found on Wikipedia at: Bicycle performance.

6.   Why is this calculator so much simpler than all others online?

As mentioned in the Wikipedia article discussing the formula above, "...simplifying assumptions..." have been made (and of necessity always will be made) by all such calculators, so here we jumped right to the heart of the matter without offering blinding distractions from minor details of questionable utility.


7.   Would the watts to mph work in an Excel Spreadsheet?

Sure, here is the formula:


See question 5 for the source of the formula above.

Constants used in the formula above are:

9.8065  = constant for gravity
.0053  = lump constant for frictional losses (chain, tires, etc)   
0.185  = lump constant for aerodynamic drag
0.621371192  = conversion of kilometers to miles


8.   Why are race categories calculated differently for men and women?

Your guess is as good as mine, because nobody has ever shown convincing evidence there is a substantial difference between the athletic potential of men compared to women.

Women are certainly capable of competing with men, so why waste time leading riders astray?




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