How to Measure Bicycle Wheel Circumference For A Cycling Computer
How to Measure a Bicycle Wheel
Three Methods:
In order to purchase a spare or replacement tire for your bicycle, you first need to determine the size of your bicycle wheels. In fact, measuring the wheels is a standard component of bicycle maintenance. Although measuring a bicycle wheel, including determining the size of both the tire and the rim, can be accomplished by either of two methods, both are simple to complete. Sometimes you might even need to know the circumference of a bicycle wheel, which is also a simple determination that can be achieved in multiple ways.
Steps
Measuring the Tire Using the Standard Method

Stand the bike upright by leaning it against a wall or using the kick stand.When the bicycle is upright, you will be able to measure the bike wheel without the bike tipping onto you. If you are measuring the bike alone, a retractable metal tape measure will be sturdier than a plastic tape measure while leaving you a free hand.

Measure the distance in inches from the ground beneath the tire to the center point of the wheel.This measurement is the wheel's radius, or half of its diameter. Multiply the length by two in order to calculate the tire's diameter. With the exception of BMX models, most adult bicycle wheels are between 26 and 29 inches in diameter.

Measure the flat part of the tire across the tread from one side to the other.The distance is the tire's width. It can vary greatly depending on the tire's intended use. Generally, the wider the tread, the rougher the intended terrain, while narrower treads promise a smoother, faster ride.

Remember when purchasing a new tire that traditional, or standard sizes place the diameter first, followed by the width.For example, a tire sized 26 x 1.75 would have a 26inch diameter and a 1.75inch width across the tread.
Measuring the Tire Using the ISO Method

Recognize if your bike's wheels are measured by the International Organization for Standardization system instead of the standard method.The ISO measuring system uses millimeters to signify the size of your bicycle's wheel. If you don't have a measuring tape with metric rules, just be aware that there are 25.4 millimeters in one inch and determine the width with a calculator, multiplying inches x 25.4.

Ensure the bike is propped up against the wall or on its kickstand.Measure in millimeters from the center of the wheel to only the inner edge of the tire. Once again, double the figure to determine the diameter. Most adult bicycle tires measured with the ISO system range from 650 to 700 millimeters in diameter.

Measure the tire's width in millimeters.Use the same process listed above, across the tread from one side to the other. Keep in mind that tires of various widths can be used on the same bicycle wheel, as long as the difference is not too drastic.

Remember when purchasing a new tire measured by the ISO system, the width is listed first, followed by the diameter.For example, a bicycle wheel sized 53.3 x 700 would have tread 53.3 millimeters wide and a wheel with a 700millimeter diameter from the inner edge of one side of the tire to the other.
Calculating the Circumference of a Bicycle Wheel

Measure the circumference, or distance around the outside of a bicycle wheel in order to properly calibrate a bicycle speedometer, odometer, GPS or computer.Just as your car's speedometer and odometer will provide inaccurate data if you change the size of its wheels, bicycle equipment must also be set based on the size of your tires.Whether you've just purchased a cyclometer or you need to calibrate an existing one because you've switched to tires with a different sized tread, you will need to determine the wheel's circumference.

Calculate circumference easily by multiplying the tire's diameter by pi.The circumference of any circle can be determined quickly if you already know the diameter of the tire from the outer edge of one side to the other. As pi is equal to 3.14, the circumference of a 26inch tire can be reached by multiplying 26 x 3.14 to equal a circumference of 81.64 inches.
 If you already know the tire's diameter and width, you can also look up the circumference on various online charts.

Measure the circumference with a string.If you don't already know the wheel's diameter, you can still measure its circumference by evenly wrapping a string around the outside edge of the tire. Mark or cut the string at the spot where it reaches back to its starting point and measure its length to determine the circumference.

Add a dot of wet paint to a spot on the bicycle tire's tread.Carefully push the bicycle in a straight line for at least two rotations, ensuring the paint marks the ground twice. Measure the ground from the first spot of paint to the next in order to determine the tire's circumference.
Community Q&A

QuestionHow do I remove the paint dot from my garage floor?wikiHow ContributorCommunity AnswerUse brake fluid. Let it soak for a few minutes, then scratch it off with your nail or a rough sponge, SOS pad, etc. Paint thinner will work as well.Thanks!

QuestionHow many pounds of pressure should there be in a bike tire?wikiHow ContributorCommunity AnswerProper tire pressure lets your bike roll quickly, ride smoothly, and fend off flats. Narrow tires need more air pressure than wide ones: Road tires typically require 80 to 130 psi (pounds per square inch); mountain tires, 30 to 50 psi; and hybrid tires, 50 to 70 psi.Thanks!

QuestionCan I replace a 700/25 tire with a 700/26 on a road bike?wikiHow ContributorCommunity AnswerAlmost definitely. The only real issue when going to a wider tire is clearance around the frame, but such a small difference shouldn't be a problem.Thanks!

QuestionWhat can I do to calibrate my bicycle computer if it has 27.5 inch wheels?wikiHow ContributorCommunity AnswerPut the bike on a flat surface, like your garage floor. Roll the wheel until the valve stem is at the bottom. Mark the floor beside the valve stem. Roll the bike straight one revolution until the valve stem is at the bottom again, and mark the floor. Measure between the marks; inches x 25.4 will give you mm. Most cycle computers want to know the mms traveled per revolution. To reset the computer, press the buttons in for 10 seconds, or remove and reinsert the battery.Thanks!

QuestionHow do I measure rim width?wikiHow ContributorCommunity AnswerUse a ruler or measuring tape. If you only have a ruler, then measure the diameter and multiply by 3.14.Thanks!

How do I measure a cycle wheel if there is no tire on it?

How to determine bicycle size?
Video
 When measuring the diameter of the wheel, prevent the tire from rotating, as this could reduce the accuracy of your measurement.
 Tire sizes are usually printed on the sidewall of the tire, expressed as diameter x width, 27x1.5, for example. 27x1.5 isn't always the same size as 27x1 1/2.
 When measuring via the standard method, the diameter should be a whole number. If you calculate a fraction, round to the nearest inch.
Things You'll Need

Measuring tape

Calculator

Bicycle or bicycle wheel/tire
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Video: Bicycle Parts  wheel and tires size
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