Cassette and Chainring Guide

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Gearing is the one part of a bicycle that is genuinely, completely knowable from published numbers. There is no ride feel here, no marketing, nothing that requires a lab. A gear ratio is teeth on the front divided by teeth on the back. That’s it. Everything else on this page is arithmetic on numbers that Shimano and SRAM print themselves.

So we’re going to do two things. First, show you what the ratios actually work out to, with the inputs on screen so you can check us. Second — and this is the part almost nobody covers — show you the two numbers Shimano publishes for every derailleur that tell you whether a cassette will physically work on your bike before you buy it.

The only two sums that matter

Ratio = chainring teeth ÷ sprocket teeth.A 50T chainring on an 11T sprocket is 50 ÷ 11 = 4.55. Every pedal turn drives the wheel round 4.55 times. A 34T chainring on a 34T sprocket is 34 ÷ 34 = 1.00 — one wheel turn per pedal turn, which is what you want on something steep.

Range = highest ratio ÷ lowest ratio.This is the number that describes the bike rather than one gear. A 50-34T chainset with an 11-34T cassette: (50÷11) ÷ (34÷34) = 4.55 ÷ 1.00 = 4.55, usually written as 455%. That is the whole spread from your easiest gear to your hardest.

Those two sums replace an enormous amount of shop-floor argument. And they let you do something useful: compare a Shimano setup with a SRAM one, which is otherwise impossible because the two brands don’t share a single tooth count.

What the range actually costs you

Here is the trade nobody states plainly: range and close ratios are the same resource, spent differently. A cassette has a fixed number of sprockets. Spread them wider and you get an easier climbing gear and bigger jumps between gears. Pack them tighter and you get smaller jumps and a worse day on a hill. There is no version where you get both, and any marketing that implies otherwise is describing a 13th sprocket.

Shimano publishes the exact sprocket sequence of its cassettes, which makes the trade visible rather than theoretical. From its own spec table, the 12-speed Ultegra cassette in 11-30T runs:

11-12-13-14-15-16-17-19-21-24-27-30T

And the same cassette in 11-34T runs:

11-12-13-14-15-17-19-21-24-27-30-34T

Look at what buying the extra range cost. The 11-30T has single-tooth steps all the way from 11 to 17 — 11, 12, 13, 14, 15, 16, 17. The 11-34T drops the 16T entirely and jumps 15 straight to 17. That is the price of the 34T sprocket, paid in the middle of the cassette where you spend most of your time on flat roads. Everything else in the sequence is identical.

That is the whole range debate, settled with a published sprocket list. You are not buying “more range”. You are trading one middle sprocket for one big one.

The capacity sum — will it even fit?

This is the part that saves you money, and it rests on two figures Shimano publishes for every rear derailleur and that we have never seen quoted in a buying guide: total capacity and max. front difference.

Total capacity is how much chain slack the derailleur can swallow. You calculate what you need like this:

Capacity needed = (big ring − small ring) + (big sprocket − small sprocket)

Work a real example. A 105 RD-R7000-SS is published with a total capacity of 35T, a maximum low sprocket of 30T and a max front difference of 16T. Put a 50-34T chainset and an 11-30T cassette on it: (50 − 34) + (30 − 11) = 16 + 19 = 35T.

Exactly 35. That setup sits precisely on the published limit — which tells you something rather elegant: Shimano designed the short-cage 105 derailleur for exactly that build and no more. The front difference is 16T, which is exactly the published maximum too. The spec sheet is describing the intended bike.

Now try to fit an 11-34T cassette to the same derailleur. The maximum low sprocket is published as 30T. 34 is more than 30. It doesn’t fit, and no adjustment fixes that— it’s a cage-length limit, not a tuning problem. You need the GS (medium cage) version, which Shimano publishes at 39T total capacity and a 34T maximum low sprocket.

This is the single most useful thing on this page. Before you buy a cassette, look up your derailleur’s published maximum low sprocket. If your intended big sprocket is larger, stop. If it fits, run the capacity sum. If the answer exceeds the published total capacity, stop. Two lookups and one addition, and you will never buy a cassette that doesn’t work.

If your shifting is poor and the cassette iswithin spec, that’s adjustment, not capacity — see how to adjust a rear derailleur.

What your tier actually lets you have

Range isn’t only limited by physics. It’s limited by what your groupset tier is offered in, and the differences are larger than the weight differences everyone argues about.

On Shimano 12-speed, the Ultegra cassette is published in 11-30T and 11-34T. The 105 cassette is published in 11-34T only. So the tighter block simply does not exist at 105 in 12-speed, at any price. Meanwhile the 11-speed tiers are far more generous: Ultegra’s CS-R8000 is listed in 11-25T, 11-28T, 11-30T, 11-32T, 12-25T and 14-28T; 105’s CS-R7000 in 11-28T, 11-30T, 11-32T and 12-25T. Going to 12-speed gained you a sprocket and lost you most of the catalogue.

On the front, Shimano publishes the 12-speed Ultegra crankset in 46-36T, 50-34T and 52-36T, and the 12-speed 105 in 50-34T and 52-36T. The 46-36T — a genuinely useful, small-gear-friendly combination — is Ultegra and above only. Dura-Ace adds 54-40T on top, for people who are not reading a buying guide.

SRAM plays a different game entirely, because its cassettes start at a 10T sprocket. Force is published in 10-28T, 10-30T, 10-33T and 10-36T; Rival in just 10-30 and 10-36 — SRAM describes those as “a more traditional road spec” and “maximum range” respectively. And the Rival 2x chainring kit is published in 46/33T and 48/35T, where Force adds 50/37T.

That 10T sprocket is why SRAM can use smaller chainrings and still reach the same top gear. 48 ÷ 10 = 4.80, against Shimano’s 50 ÷ 11 = 4.55. SRAM’s 48T ring pushes a bigger top gear than Shimano’s 50T ring. The chainring number on the crank tells you nothing on its own — which is exactly why the division matters and the marketing doesn’t.

What we're not going to tell you

Two honest gaps, because this page would be less useful without them.

We haven’t converted any of this to gear inches or metres of development. Those figures need a wheel and tyre diameter, and your tyre’s real rolling circumference depends on width, pressure and your weight — none of which anyone publishes for your bike. Ratios are exact. Gear inches would be an estimate wearing a decimal point. So we publish the ratio and let you stop where the certainty stops.

We can’t tell you which gear you need.That depends on your legs, your hills and your loaded weight, and anyone who gives you a universal answer is guessing about your quads. What we can say is what the arithmetic shows: a 34T ring on a 34T sprocket is 1.00, and a 1.00 is a genuinely low gear for a road bike. If you are grinding at 1.00, the answer is a smaller chainring or a bigger sprocket — and now you know how to check whether your derailleur will accept one.

Where your tier sits in all this, and what the rung above buys, is in groupset hierarchy explained. If you’re running out of gears on loose surfaces rather than tarmac, gravel bike gearing explained covers the 1x case that this page deliberately doesn’t.

And whatever cassette you land on: it wears out because the chain wore out first and you didn’t notice. The cassette is the expensive half of that mistake — when to replace a bike chain.