Tyre and rim width calculator, with tubeless sealant

700 x 32 mm on a 21 mm rim

Good fit

A 32 mm tyre on a 21 mm rim is well inside the ETRTO range of 16 to 25 mm.

Nicely rounded, with the sidewalls well supported. It should come up about 32.2 mm wide on this rim. Rims from 16 to 25 mm inside suit this tyre.

Sealant

Each tyre	65 ml (2.2 oz)
First time in a new tyre	95 ml (3.2 oz)
Both wheels, first fit	190 ml
Both wheels, each refresh	130 ml

Silca say to add an extra ounce (about 30 ml) the first time in a new tyre.

Topping up. Silca say put fresh sealant in every 6 months, and top up after any puncture that lost sealant. In between, hold the wheel by your ear and give it a shake. If you can't hear it slosh, it needs more.

From Silca's own fill guide. Sizes between their chart sizes are worked out from the two nearest.

Why I made this

Two questions come up every time I set up a wheel. Will this tyre work on this rim, and how much sealant goes in? Get the first wrong and the tyre sits like a lightbulb or gets stretched flat. Get the second wrong and a thorn lets the air out, or you carry a puddle round the trail. So I've put both answers in one place.

Where the numbers come from

The fit comes from the ETRTO chart, the tyre and rim standard every maker works to, as Continental publish it. It lists the inside rim widths that suit each tyre width. How wide the tyre comes up uses Continental's rule of 0.4 mm of tyre for each 1 mm of rim. For Nextie rims I also check Nextie's own tyre range for that rim.

Silca's sealant figures come from their own fill guide, which goes from 55 ml for a 700 x 28 up to 135 ml for a 29 x 2.6. Squirt's come from their own instructions, which give a range for road, gravel, 26 and 27.5 inch, and 29 inch tyres. Between their sizes I work it out from the two nearest. Neither lists fat bike tyres, so for those I use Stan's advice of 6 to 7 oz a tyre. I haven't used anyone else's calculator figures.

Setting up a tubeless tyre

Clean the inside of a new tyre first. Squirt say the release agent from the factory can upset the sealant.
Shake the bottle hard. The bits that do the sealing sink to the bottom.
Put the first-fit amount in, seat the tyre, then spin and tilt the wheel so it coats the whole inside.
Ride it soon. Riding spreads the sealant better than leaving the bike in the shed.
Check it after a week or two. A new tyre will have soaked some up.

Want it done for you?

I fit tubeless tyres with sealant in the workshop here in Rochford. See the workshop price list, or pick a tyre from tyres & tubes. If your tyres and rims don't match, I build tubeless ready wheels to order on Nextie rims in the width you need, and the tyre pressure calculator will help you set them up.

Tyre fit and sealant questions

Will a 28 mm tyre fit a 25 mm rim?

Not by the ETRTO chart. A 28 mm tyre suits rims from 15 to 23 mm inside. On a 25 mm rim it gets pulled square and the rim edge sits out in the open. A 30 or 32 mm tyre suits that rim better.

What rim width for a 2.4 inch MTB tyre?

ETRTO list 21 to 35 mm inside for a 2.4. Around 28 mm, the middle of that, it sits nicely rounded with good support in corners. On 21 mm it'll be tall and squirmy at trail pressures.

Does rim width change how wide my tyre is?

Yes. Continental say the tyre gets about 0.4 mm wider for every 1 mm of extra rim width. Their own example is a 28 mm tyre coming up 26.4 mm on a 15 mm rim and 29.6 mm on a 23 mm rim. Worth knowing if your frame is tight.

What is a lightbulb tyre?

A wide tyre on a narrow rim. It pinches in where it meets the rim and balloons out above it, like a lightbulb. It feels vague and can fold over in corners, so you end up running more pressure than you'd like.

Do hookless rims take any tyre?

No. Hookless rims need tubeless ready tyres from the rim maker's approved list, and road and gravel hookless rims are limited to 5 bar (72.5 psi). Check both before you fit anything.

How much sealant do I need for a road tyre?

Silca say 55 ml for a 700 x 28 and 65 ml for a 32. Squirt say 60 to 80 ml for road tyres. Add a bit more the first time in a new tyre, because some soaks into the casing.

How much sealant for a 29er MTB tyre?

Silca say 115 ml for a 29 x 2.25 and 125 ml for a 2.4. Squirt say 100 to 120 ml for 29 inch tyres. Big, open trail tyres with thin casings like the top end of that.

Why do I need more sealant in a new tyre?

A new tyre soaks some up in the first few rides while it coats the inside and seals the casing. Silca say to add an extra ounce (about 30 ml) the first time. Squirt say check after 1 to 2 weeks and top up.

How often should I top up tubeless sealant?

Silca say put fresh in every 6 months. Squirt don't give a set time: open the tyre now and then, and if it has gone thin and see-through, tip it out and put fresh in. Hot weather and a lot of riding dry it out quicker. After a puncture that sprayed sealant, top up straight away.

Can I mix two different sealants?

I wouldn't. Some react with each other and go lumpy. Silca even say not to mix their new sealant with their old one. Clean the old stuff out first, then put the new one in.

Can I put sealant in through the valve?

Yes. Take the valve core out, use the bottle's nozzle or a syringe, then put the core back and pump it up. Turn the valve to 4 or 8 o'clock first so the sealant doesn't sit in the valve. Squirt's 1 litre bottle goes in with the tyre off the rim, because of the BeadBlock bits.

Tyre fit and sealant calculator