Your gear ratio is the single number that explains why one bike feels effortless on a climb and another feels like wading through treacle on the flat. It is also one of the easiest things to work out yourself: you need nothing more than the ability to count teeth and do a little division. Once you can calculate a ratio, you can translate it into gear inches and metres of development, build a full gear chart for your cassette, spot the gears you never actually use, and choose sensible gearing for the riding you do. This guide takes you through all of it, step by step, with worked examples for both single-speed and derailleur bikes.
Step 1: count the teeth
Every gear calculation begins with two tooth counts:
- Chainring — the front ring(s) attached to your cranks. Count the teeth, or read the number stamped on the ring (e.g. 50, 34, 42).
- Cog (sprocket) — the rear sprocket the chain is on. On a cassette these range from a small high-gear cog (often 11 or 12 teeth) to a large low-gear cog (28, 34, even 52 on wide-range setups).
Count carefully — it is easy to be one or two teeth out, and that error carries through every subsequent number. If a ring or cog is worn or unmarked, count twice.
Step 2: compute the ratio
The gear ratio is simply chainring teeth divided by cog teeth:
- Gear ratio = chainring ÷ cog
A 50-tooth chainring with a 25-tooth cog gives 50 ÷ 25 = 2.0. That means the rear wheel turns twice for every single turn of the pedals. A 34-tooth ring with a 34-tooth cog gives 34 ÷ 34 = 1.0 — one wheel turn per pedal turn, a classic low climbing gear. The higher the ratio, the harder the gear and the faster you go for a given cadence; the lower the ratio, the easier the pedalling and the better for climbing.
The ratio alone is useful for comparing gears, but it ignores wheel size — and a given ratio on a 26-inch wheel moves you less far than the same ratio on a 700c wheel. That is where gear inches and development come in. For the deeper theory behind these measures, see gear inches, ratios and development explained.
Step 3: convert to gear inches and development
Gear inches express your gear as the diameter (in inches) of an equivalent direct-drive wheel — a hangover from the penny-farthing era that remains the most popular comparison number among cyclists. The formula is:
- Gear inches = ratio × wheel diameter in inches
A 700c wheel with a typical road tyre has an outer diameter of roughly 26.4 inches (about 670 mm). So our 2.0 ratio becomes 2.0 × 26.4 ≈ 52.8 gear inches. Road gears commonly span from the high 20s (easy climbing) to well over 100 gear inches (flat-out sprinting).
Development (or "rollout") tells you how far the bike travels per pedal revolution — handy because it is an intuitive real-world distance. Multiply the ratio by the wheel's rolling circumference:
- Development = ratio × wheel circumference
With a 700x25c wheel circumference of about 2.10 m, our 2.0 ratio gives 2.0 × 2.10 = 4.20 m per pedal stroke. Because wheel size feeds directly into both numbers, getting your tyre and wheel data right matters — see understanding bike wheel sizes to pin down your exact diameter and circumference.
Worked example: a single-speed
Single-speeds are the simplest case because there is only one gear to think about. Say you run a 46-tooth chainring with a 16-tooth cog on 700c wheels:
- Ratio = 46 ÷ 16 = 2.875
- Gear inches = 2.875 × 26.4 ≈ 75.9
- Development = 2.875 × 2.10 ≈ 6.04 m per pedal stroke
That is a versatile all-round single-speed gear: tall enough to hold speed on the flat, not so tall that you stall on gentle climbs. Want it easier for a hilly area? Drop to a 17 or 18-tooth cog (yes, a bigger cog lowers the ratio), or fit a smaller chainring.
Worked example: a derailleur bike
A geared bike is just many single-speed calculations stacked together. Take a 2x road setup with 50/34 chainrings and an 11-28 cassette. To find any gear, divide the chosen ring by the chosen cog. The hardest gear is 50 ÷ 11 = 4.55 (≈ 120 gear inches); the easiest is 34 ÷ 28 = 1.21 (≈ 32 gear inches). Every other combination falls between those two extremes.
Building a full gear chart
To see your whole range, build a table. Down one side list your cassette cogs (11, 12, 13, 14, 15, 17, 19, 21, 24, 28); across the top list your chainrings (34 and 50). Fill each cell with chainring ÷ cog, then optionally convert each to gear inches. A handful of cells for our example:
- 34 × 28-tooth cog → ratio 1.21 → ≈ 32 gear inches (easiest)
- 34 × 17 → 2.00 → ≈ 53 gear inches
- 50 × 17 → 2.94 → ≈ 78 gear inches
- 50 × 11 → 4.55 → ≈ 120 gear inches (hardest)
Rather than fill the grid by hand, the gearing tool on the Convert.bike home page generates the whole chart for you — enter your chainrings, cassette and wheel size and it returns ratios, gear inches and development for every combination at once. Pair it with our bike speed, cadence and gearing calculator to see what speed each gear gives at your favourite cadence.
Identifying duplicate and overlapping gears
Sort all your computed gear inches into a single ascending list and you will quickly spot two things. First, duplicates: gears so close in value they are effectively the same. In our example, 50/24 (≈ 55 gear inches) and 34/17 (≈ 53 gear inches) sit almost on top of each other — usable, but redundant. Second, overlap: the easier gears of the big ring overlap the harder gears of the small ring, which is why a 20-speed bike has nowhere near 20 distinct gears. This is normal and not a fault; it just means a couple of cross-chain combinations are best avoided in favour of the cleaner chainline alternatives.
Choosing gearing for your riding
With the numbers in front of you, you can tune your gearing deliberately:
- Hills: prioritise a low easiest gear. Aim for something around or below 30 gear inches (e.g. a 34-tooth ring with a 32 or 34 cog) so you can spin up steep gradients without your cadence collapsing.
- Flats and speed: prioritise a tall top gear, roughly 100+ gear inches, so you are not spinning out on fast descents or in a tailwind.
- Single-speed: there is no compromise to spread across a cassette, so pick one ratio that matches your typical terrain — something in the 65-75 gear-inch range suits most mixed riding.
The key is to match the spread and the extremes to where you actually ride, not to chase the widest range for its own sake. For the underlying method of turning ratios into a full picture, the dedicated guide on gear inches and development is worth a read.
Next steps
Grab your bike, count your chainring and cog teeth, and work out a single ratio by hand to get the feel of it. Then convert that ratio to gear inches and development so you know what it means on the road. When you are ready for the full picture, feed your chainrings, cassette and wheel size into the home-page gearing tool to build a complete chart, hunt down the duplicate and overlapping gears, and confirm your easiest and hardest gears suit your terrain. Cross-check your wheel and tyre figures with our wheel-size guide so every number is accurate, and you will understand your drivetrain better than most riders ever do.