The basics

Before you start

• Measure wear first: see Measure chain wear. A chain that's still good doesn't need replacing.
• Buy the right chain: count the sprockets on the cassette (9, 10, 11 or 12 speeds). A 12-speed chain is narrower than a 9-speed one: they aren't interchangeable. Some recent drivetrains need their own chain (for example the "T-Type" chains of SRAM Eagle Transmission): the drivetrain's manual says so.
• This guide is for derailleur bikes. On a single-speed or hub-gear bike, chain length and tension are set differently (wheel position or a tensioner): ask a shop, or follow the bike's manual.

How chains are joined

A chain closes in one of two ways, depending on make and model:

• A quick link (Missing Link, PowerLock): two half-links that snap together, no tool needed to close it. Many are single-use: SRAM's 10, 11 and 12-speed links, KMC's 12-speed links, and every link on KMC e-bike chains. Once opened, fit a new one.
• A connecting pin: a special pin, specific to the chain's make and model, pressed in with a chain tool. A regular pin pushed out and back in won't hold.

Steps

1. Note the chain's path

Look at how the old chain runs through the rear derailleur, between its two jockey wheels, and around the cage tabs (the small plates that keep the chain in place). Take a photo: the new one must follow exactly the same path.

2. Open the old chain

• With a quick link: bring it to the lower run, squeeze the two links around it together with quick link pliers, and it pops apart.
• Without a quick link, or if it's seized: the chain is worn, so it doesn't matter where you break it. Set it in the chain tool, away from any old connecting pin, and drive a pin out, keeping the tool straight.

Take the chain off the bike.

3. Find the right length

This is the step that matters. A chain that's too short can rip the derailleur off in the big chainring and big sprocket; too long, it sags and skips in the small chainring and small sprocket.

If the old chain was the right length, lay both chains side by side and count pin by pin, rather than measuring with a ruler: the old one has stretched. Line them up by their outer plates, or by their inner plates, and cut the new one to the same number of links.

Otherwise, measure on the bike:

• Cassette up to 36 teeth (road, city, gravel with two chainrings): wrap the chain around the big chainring (1) and the big sprocket (2), through the front derailleur but not through the rear derailleur. If your chain uses a quick link, fit half of it on the end: it counts in the length. Find the nearest pin where both ends meet, inner plate to outer plate (3), then add two pins, that is one inch (4).
• Big sprocket of 42 teeth or more (mountain and gravel bikes with a single chainring): same method, still bypassing the rear derailleur, but add four pins, that is two inches.
• Full-suspension mountain bike: the chain path changes as the suspension compresses. Measure with the suspension fully compressed: that means removing the shock, or letting the air out of an air shock. If you're not sure how, leave this step to a shop.
• SRAM Eagle Transmission (T-Type chain): SRAM gives the number of links in a chart, based on chainring size and chainstay length (the distance from the bottom bracket axle to the rear axle, suspension extended). Follow that chart rather than the method above.

4. Cut the chain

Set the chain in the chain tool, with the pin to remove right in front of the tool's pin, and turn it in straight. With a quick link, both ends must finish on an inner link (narrow plates): the quick link fits between them. With a connecting pin, one end finishes on an inner link and the other on an outer link.

5. Thread the new chain

• Chain direction: some chains only go on one way. Logos and markings face outwards, towards you when you look at the drive side.
• Run it around the chainring, through the front derailleur, then through the rear derailleur following your photo: around both jockey wheels and on the correct side of each cage tab.
• Bring both ends together on the lower run, under the chainstay.

6. Join the chain

• With a quick link: if it has arrows, they show the direction of travel. Fit both halves together, then pull the neighbouring links apart with the pliers to seat it. Without pliers: move the link to the upper run, hold the rear brake and push down firmly on the pedal. Check that it's fully engaged on both sides.
• With a connecting pin: oil it, then press it in with the chain tool from the inside of the bike outwards. Stop when it sticks out as far as its neighbours, then snap off the tapered guide section.

7. Check the length and the links

• Big chainring and big sprocket: the chain makes a slight bend at each rear derailleur jockey wheel, without being pulled straight.
• Small chainring and small sprocket: the chain doesn't sag, and doesn't rub on itself in the derailleur.
• Backpedal slowly while watching the chain go through the derailleur: a link that makes the jockey wheel jump is a stiff link. Flex it gently side to side to free it up.

8. Lube it

A new chain comes greased from the factory: don't degrease it. Just wipe the outside, then lube it as usual: see Clean and lube your chain.

Check it's done right

• Every gear shifts, both up and down the cassette.
• Pedalling hard, standing up, in each sprocket, the chain doesn't skip.
• The quick link, or the connecting pin, sits flush with the neighbouring links.

Common mistakes

• Cutting too short: you can't lengthen a chain without an extra link. When in doubt, leave it one link longer, try it, then shorten.
• Reusing a single-use quick link, or a regular pin: the chain can come apart while riding.
• Missing a cage tab: the chain rubs and shifting suffers.
• Fitting a directional chain backwards: shifting gets worse.

What next

Log the replacement and the distance in your logbook: the next wear check is due in about 1,000 km. If you always replace the chain in time, the cassette often lasts two or three chains.

Sources

This guide was checked against the following documents:

• Park Tool, Chain Replacement: Derailleur Bikes (opening, direction, joining, stiff links)
• Park Tool, Chain Length Sizing (two and four pins, full suspension, checks)
• KMC, Connectors (reusable or not, e-bike chains)
• KMC, How to treat a new chain (factory grease)
• BikeRadar, Chain quick links (reusing quick links)
• SRAM, Eagle AXS Transmission T-Type Chain Length (chart, document GEN.0000000006479, copy hosted by a retailer)

Values vary between models: your part's manual always takes precedence.