Tire Circumference for Bike Computers: 700×25C and 28C Values and How to Measure

Tire circumference is the yardstick for speed and distance

A speed sensor on the wheel or hub only counts how many times the tire has turned. The bike computer multiplies that count by the set tire circumference (the distance the tire travels per revolution) to get distance, then divides by time to get speed. In other words, the circumference setting is the very yardstick that measures speed and distance.

• Distance = tire revolutions × tire circumference
• Speed = revolutions per second × tire circumference × 3.6 (also ÷ 1000 when using mm)
• If the circumference is set 1% short, speed and distance both read 1% low

1% sounds small, but it's 1 km on a 100 km long ride and 50 km for someone who rides 5,000 km a year. If you compare times with past rides on Strava or in training logs, or track maintenance by distance (such as replacing the chain every 3,000–5,000 km), it's worth getting it right once at the start.

Reading the ETRTO (28-622) on the sidewall

Besides a name such as "700×28C," a tire's sidewall carries a number such as "28-622." This is the ETRTO marking: the first number is the tire width (mm), and the second is the bead seat diameter (the diameter where the tire seats on the rim, mm). Names vary by country and maker, but ETRTO gives the actual dimensions, so use it when calculating circumference.

Note that "26-inch" tires differ by as much as 31 mm in bead seat diameter between MTBs (559) and city bikes (590), and "20-inch" comes in two sizes, 406 and 451. Choosing a circumference by the inch figure alone can put you off by 5% or more, so always check the second ETRTO number on the sidewall. The origin of inch sizes is also touched on in Converting Inches to cm, and Reading Bike Sizes.

The formula: π × (bead seat diameter + width × 2)

A tire's outer diameter is the bead seat diameter plus two tire heights, one at the top and one at the bottom. Road and cross bike tires are close to round in cross section, so height can be treated as roughly equal to width. So outer diameter ≈ bead seat diameter + width × 2, and multiplying by π gives an estimated circumference.

1. 1

   Read the ETRTO

   A 700×28C tire says "28-622" on the sidewall: width 28 mm, bead seat diameter 622 mm.

2. 2

   Find the outer diameter

   622 + 28 × 2 = 678 mm. This is the estimated outer diameter of the tire.

3. 3

   Multiply by π

   678 × 3.1416 ≈ 2130 mm. Enter 2130 in the bike computer.

The formula also shows that wider tires have a longer circumference. Each extra 1 mm of width adds 2 mm to the outer diameter and about 6.3 mm to the circumference. Going from 25C to 28C lengthens the circumference by 19 mm, or about 0.9%, so remember to update the setting when you change tires.

List of tire circumferences (calculated from ETRTO)

These circumferences are calculated as π × (bead seat diameter + width × 2). Enter these values for the initial setup, and if you want more precision, fine-tune with the roll-out measurement described below.

Interestingly, 650×47B and 700×28C both come to about 2130 mm. The circumference itself shows that "650B with wide tires" on a gravel bike is a combination that brings the outer diameter close to a 700C road tire so the handling doesn't change.

Why do maker tables differ by a few mm?

Circumference tables in bike computer manuals and on maker websites can differ from the calculated values above by a few mm, with 700×25C around 2105 mm and 700×28C around 2136 mm, for example. Neither is wrong; real tire circumference varies for the following reasons.

• Marked versus actual width: even a "28C" tire bulges 1–3 mm wider on a rim with a wide inner width. A 19C and a 23C rim change the circumference by a few mm
• Differences by model: casing and tread thickness and cross-section shape vary by maker, so outer diameters don't match even with the same marking
• Air pressure and rider weight: when ridden, the tire's contact patch flattens, and the distance actually traveled tends to be about 0.5–1% shorter than calculated
• Wear: as the tread wears down, the circumference gets shorter too

In terms of error size, the difference between calculated and table values is around 0.3%. You'd hardly feel it: it's about 0.1 km/h at 30 km/h. But if you want to compare data with other people or match your GPS records, a value measured with your own tire, pressure and weight is the most accurate.

The most accurate measurement: the roll-out

A roll-out measures the distance of one revolution by actually rolling the tire. With a tape measure and masking tape or chalk, it takes 5 minutes. Measuring just one revolution makes a 1–2 mm misreading the full error, so the trick is to measure several revolutions together and divide.

1. 1

   Set your usual air pressure

   Pump up to the pressure you normally ride at. Measuring at low pressure gives a short circumference.

2. 2

   Put the valve at the bottom and mark the floor

   Stop the wheel fitted with the speed sensor (usually the front or rear hub) with its valve straight down, and mark the tire's contact point on the floor with tape.

3. 3

   Roll straight with your weight on the bike

   If possible, straddle the bike with your weight on it and push it straight ahead. One revolution ends when the valve is straight down again; roll for 3–5 revolutions.

4. 4

   Measure the distance and divide

   Use a tape measure from the first mark to the final position, and divide by the revolutions. If 5 revolutions cover 1065.0 cm, 10650 mm ÷ 5 = 2130 mm.

How far off do speed and distance get?

Displayed speed and distance are off by the ratio of the set circumference to the actual circumference. Set too short, they read slow and short; set too long, they read fast and long. Here's how large the difference gets with common setting mistakes.

Forgetting to update the circumference after changing tires, or mixing up 20-inch 406 and 451, happens more often than you'd think, so check the settings screen once after a tire change. Circumference affects not only speed but also, in the same proportion, the speed calculated from gear ratio, such as how fast you go at a cadence of 90 rpm. Turning a 50×17 gear at 90 rpm gives 33.5 km/h on 25C (2111 mm) and 33.8 km/h on 28C (2130 mm).

What to look up next

• See the relationship between cadence and speed in a table: Cadence and Speed Chart: How Fast Is 90 rpm?
• Unsure about chainring sizes (50-34, 52-36): Compact vs. Semi-Compact Cranks
• Considering an 11-34 cassette: How to Choose a Cassette: 11-30 vs. 11-34
• Gear ratios and skid patches on a fixed-gear bike: How to Choose a Fixed-Gear Ratio: 48×17 and Skid Patches

FAQ

What is the circumference of a 700×25C tire in mm?

The value calculated from ETRTO (25-622) is about 2111 mm. Maker tables often show around 2105 mm, a difference of about 0.3%. For a precise setting, measure with a roll-out, rolling the tire with your weight on the bike.

What circumference should I set for 700×28C?

The calculated value is about 2130 mm (π × (622 + 28 × 2)). On wheels with wide rims the tire bulges wider, so a measurement may come to about 2130–2140 mm.

What is the tire circumference formula?

Circumference ≈ π × (bead seat diameter + tire width × 2). With the ETRTO marking "28-622" on the sidewall, 3.1416 × (622 + 56) ≈ 2130 mm.

What goes wrong if the circumference setting is off?

Speed and distance are off by the same proportion. A 1% error means 1 km over 100 km. If you change to 28C but leave the 25C setting, it reads about 0.9% slow and short.

Do city bike and MTB 26-inch tires have the same circumference?

No. City bikes (WO standard) have a bead seat diameter of 590 mm, and MTBs (HE standard) 559 mm. 26×1-3/8 has a circumference of about 2086 mm and 26×1.95 about 2070 mm, and their widths also differ. Always check the ETRTO on the sidewall.