What size bike do I need?

By CrankMart

A bike that fits is a bike you'll want to ride. The good news: finding your size takes two measurements, a few minutes and one maker's size chart. Here's how.

1. Measure your height and inseam

You only need a wall, a tape measure and a hardback book.

1. Height: stand against a wall in the shoes you'll ride in and measure from the floor to the top of your head.
2. Inseam (inside leg): stand with your back to the wall, feet about shoulder-width apart. Slide a thick book between your legs, spine up, and lift it gently as if it were a saddle. Mark where the top of the spine touches the wall, then measure from the floor to the mark.

This book-against-the-wall method is the one Giant describes in its own sizing guide (source G7 below).

Why both? Height gets you to a starting row on a size chart. When you land between two sizes, inseam helps you choose. Many makers list both on their charts.

2. Know how bikes are sized

• Mountain bikes are mostly sold in letter sizes: XS, S, M, L, XL (some makers add in-between sizes such as ML, or 2XL).
• Road bikes are often sized in centimetres, such as 52, 54 or 56. Some newer ranges use letters instead.
• Gravel bikes use either, and some makers show both (for example "M (52–54 cm)").
• Older bikes are sometimes sized in inches.
• Kids' bikes are sized by wheel size, not frame size. See our kids' guide.

The size is often printed on the frame (look near the seat tube or the bottom bracket) or in the listing. If a used listing doesn't say, ask the seller for the model, year and size.

3. Read one maker's chart (every maker is different)

Charts differ from maker to maker, and even between models from the same maker. That's why we show each chart with its source and never as one "universal" table.

Example: Trek's chart for the Marlin 6 Gen 3 (hardtail mountain bike)

Size	Rider height	Inseam
XS	145–155 cm	69–73 cm
S	155–165 cm	74–78 cm
M	165–177 cm	79–83 cm
ML	173–180 cm	81–86 cm
L	177–188 cm	84–89 cm
XL	188–195 cm	89–91 cm
2XL	195–203 cm	91–97 cm

Source: Trek, Marlin 6 Gen 3 size chart, trekbikes.com (G1), accessed 31 Oct 2026. Trek's page also lists XS and S with 27.5" wheels and M and up with 29" wheels.

Example: Giant's chart for the Anthem Advanced SL 1 (cross-country full-suspension mountain bike)

Size	Rider height
S	163–172 cm
M	171–180 cm
L	179–188 cm
XL	187–198 cm

Source: Giant, Anthem Advanced SL 1 sizing guide, giant-bicycles.com (G4), accessed 31 Oct 2026. Giant notes its sizing is "to be used as a reference only" and suggests checking with a Giant dealer.

Spot the difference: a Trek Marlin in medium is listed for riders of 165–177 cm, while a Giant Anthem in medium is listed for 171–180 cm. Same letter, different ranges. Always use the chart for the exact model and year you're looking at.

Example: Trek's chart for the Domane SL 6 Gen 4 (road bike, sized in cm)

Size	Rider height	Inseam
44	150–153 cm	71–73 cm
47	152–158 cm	71–75 cm
50	158–163 cm	74–77 cm
52	163–168 cm	76–79 cm
54	168–174 cm	78–82 cm
56	174–180 cm	81–85 cm
58	180–185 cm	84–87 cm
60	185–190 cm	86–90 cm
62	190–195 cm	89–92 cm

Source: Trek, Domane SL 6 Gen 4 size chart, trekbikes.com (India site) (G2), accessed 31 Oct 2026.

Example: Trek's chart for the Checkpoint ALR 5 Gen 3 (gravel bike, letters with cm)

Size	Rider height	Inseam
XS	143–158 cm	67–74 cm
S (49 cm)	158–164 cm	74–77 cm
M (52–54 cm)	164–177 cm	77–83 cm
ML (56 cm)	177–188 cm	83–88 cm
L (58 cm)	188–194 cm	88–91 cm
XL (61 cm)	194–203 cm	91–95 cm

Source: Trek, Checkpoint ALR 5 Gen 3 size chart, trekbikes.com (G3), accessed 31 Oct 2026.

These are examples from one maker's chart each, not rules for every bike. Sizes differ between makers and models. Check the maker's size chart for the exact model and year, and take a test ride if you can.

4. Between sizes? Here's how to choose

• Use your inseam to pick between two overlapping rows.
• Mountain bikes: look at reach. Trek's own sizing guide says reach and effective top tube matter more than seat-tube length when comparing mountain bike sizes, especially across brands. You'll find them in the geometry chart on the maker's page for that model (G5).
• Think about how you ride. Trek also says riders between sizes choose by riding style and body proportions (G5). A bike shop can help you decide.
• Ride it. A short test ride tells you more than any chart. Can you reach the bars comfortably? Can you stand over the bike with both feet on the ground?

5. E-bikes

E-bikes come in frame sizes too, and each e-bike model has its own size chart on the maker's site. Use the chart for that exact e-bike model and year, the same way as above. Then read our guide to checking the motor and battery.

6. Buying used? Size checks that help

• Ask the seller for the brand, model, year and frame size so you can find the right chart.
• Look for the size printed on the frame and compare it with the listing.
• If a part has been swapped (stem, bars, seatpost), the bike may feel different from stock. Ask what's been changed.

On CrankMart Australia: filter by frame size when you browse, then message the seller through CrankMart with any sizing questions. Want a hand with fit? Find a bike shop near you in our directory.

FAQ

Is there one size chart that works for every bike? No. Makers use different ranges, even for the same letter size. Use the chart for the exact model and year.

My height says M but my inseam says L. Which one? Look at the reach on both sizes and think about how you ride. A test ride, or a chat with a bike shop, is the best tie-breaker.

Road bike sizes in cm and mountain bike sizes in letters: can I compare them? Only through each maker's chart. Some makers show both, as in the Checkpoint example above.

Are women's bikes sized differently? Some makers offer women's-specific models with their own charts. Use the chart for the model you're looking at.