Bike sizing: what size bike do I need?
The bike was 30 percent off and a size too big. What size bike do you need? How sizes work for road and mountain bikes, and why size comes before the deal.
The bike was 30 percent off. It was the right brand, the right colour and the last one in the shop. It was also one size too big, Six months later your back and shoulders are sore after every ride, and the bike mostly lives in the garage.
Frame size is the one thing a deal can't fix. Stems, seatposts and saddles can be adjusted a little, but a frame that's too big or too small never quite feels right. Below: how bike sizes work, a size chart to start from and how to check the fit before you pay.
What to decide first
Measure two numbers: your height, without shoes, and your inseam, from the floor to your crotch while standing against a wall with a book pulled up snugly between your legs. Then decide which type of bike you're buying, because road and mountain bikes use different size systems. If you haven't settled that yet, start with road, gravel or mountain bike.
How bike sizes are labelled
Road and gravel bikes are often sized in centimetres (52, 54, 56) and sometimes with letters. Mountain bikes usually use letters (S, M, L, XL). The numbers and letters aren't standard across brands, so a medium from one brand can fit like a small or a large from another. Always use the size chart for the exact bike you're looking at.
| Size | Road (frame) | Mountain |
|---|---|---|
| XS | 143 to 158 cm (47 cm) | 145 to 155 cm |
| S | 158 to 162 cm (50 cm) | 155 to 165 cm |
| M | 162 to 173 cm (52 to 54 cm) | 165 to 177 cm |
| ML | 173 to 179 cm (56 cm) | 173 to 180 cm |
| L | 179 to 188 cm (58 cm) | 177 to 188 cm |
| XL | 188 to 199 cm (60 to 62 cm) | 188 to 195 cm |
Between sizes
The ranges overlap, and plenty of riders sit on the line. Your inseam and your reach (arm and torso length) decide it. Longer legs and arms for your height point to the bigger frame. A shorter torso, less flexibility, or a preference for a more upright position point to the smaller one. The smaller frame is usually easier to adjust, because a longer stem and a higher seatpost can add reach and height, while there's only so much you can take away from a big frame.
Road and gravel: the verdict
Use the brand's chart, then check the reach on a test ride. You should reach the hoods with a slight bend in your elbows and a relaxed back, and you should be able to stand over the top tube with a little clearance. If you're new to drop bars, lean towards the size that feels less stretched. Whatever the price, check your size on the listing.
Mountain bikes: the verdict
Use the brand's chart, then stand over the bike and ride it. Modern mountain bikes are longer than they used to be, so many riders fit the size their height suggests without needing to size down. On technical trails, a bike that feels slightly smaller is often easier to move around; on long, flowing routes, a slightly bigger one can feel more stable. Check your size on the listing before the discount gets your attention.
A bike fit is worth it
A professional bike fit sets saddle height, saddle position, reach and handlebar height for your body, and many bike shops in South Africa offer one. On an expensive road or gravel bike, or if you've had back, knee or neck niggles, it's money well spent. It can also tell you which frame size to buy before you buy it. On a tight budget, a good fit often does more for comfort than paying up for a better groupset.
Where the deals are
Outgoing model years are where a lot of bike markdowns turn up (more on that in which bike for how you ride). For sizing, the point is this: the common sizes sell first, which leaves very small and very large frames on the floor longest. That's good news if it's your size and a trap if it isn't.
The Cycling Shop ranks live cycling deals from the shops we track by REALRANK™, which reads the discount, the rand saving, how the price compares to other shops, the brand, how recently the price dropped and whether it's in stock across sizes.
Before you pay
Check the size on the listing against the brand's own chart, not a generic one. Ask whether the shop will swap the stem or adjust the fit at handover. Buying online in South Africa, check the returns policy for bikes, since many shops treat a ridden bike differently from an unused one. Keep the invoice for the frame warranty and the Consumer Protection Act's six-month defect right.
Before you buy, Check The Deal. Shop great cycling deals, verified and ranked by REALRANK™.
Frequently asked questions
What size bike do I need for my height?
Use the size chart for the exact bike you're looking at. As a rough guide, riders around 165 to 173 cm often fit a medium, but brands differ.
Should I size up or down if I'm between sizes?
Longer legs and arms point to the bigger frame. A shorter torso or a preference for a more upright position points to the smaller one, which is also easier to adjust.
Are bike sizes the same across brands?
No. A medium from one brand can fit like a small or a large from another. Always check the brand's own chart.
Is a bike fit worth it?
On an expensive bike, or if you've had back, knee or neck niggles, yes. It sets the bike up for your body and can confirm the right frame size before you buy.
Where are the best bike deals in my size?
The Cycling Shop on THE REAL DEAL ranks live cycling deals from the shops we track by REALRANK™, and stock is one of its six signals. Check your size on the listing before you buy.
Sources
- Trek South Africa, Domane SL 5 size chart (road sizes by rider height), read 22 October 2026
- Trek South Africa, Marlin 5 size chart (mountain sizes by rider height), read 22 October 2026
- Consumer Protection Act, section 56, implied warranty of quality, read 19 October 2026
Written by Dean. Reviewed 27 October 2026. The size guide follows one brand's published chart and is a starting point, not a fitting. Some links earn THE REAL DEAL a commission. Placement buys visibility, it never touches REALRANK™.