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Bike Fit Calculator: Find Your Perfect Frame

With this bike fit calculator, you can estimate your ideal bike size in seconds. Choose the bike type—road, mountain, or city/trekking—enter your height and inseam, and voilà: you get frame size suggestions. You can also use it to size a child’s bike.

If you’re unsure how to measure inseam, or wonder, “What frame size do I need?”, read on — I break it down step by step. Below, you’ll also find a bike size chart for mountain and road bikes.

Why Bike Shopping Changed During the Pandemic

When public transport slowed down and physical distancing kicked in, many people turned to cycling. Bikes became the new must-have, like toilet paper, in early 2020. Sales soared, and suddenly people who never biked started thinking seriously about frames, gears, and fitting.

I include this because many readers now testing the space feel that same energy — “I want a bike, but where do I begin?”

Types of Bikes & Why Fit Differs

Before you pick a frame size, you must pick the bike type. The frame geometry and your posture change with each. Here are the main three:

Bike Type	Primary Use / Terrain	Handlebar Style	Tire / Geometry Notes
Road / Gravel	Paved roads, mixed surfaces	Drop bars	Narrow tires, more stretched geometry
Mountain (MTB)	Off-road, trails, rugged terrain	Flat bars	Wide tires, suspension, more upright fit
City / Trekking / Hybrid	Commuting, errands, casual riding	Flat or riser bars	Balanced geometry, comfort, some capability

I use “bike type,” “road bike,” “mountain bike,” and “city bike” as entities you’ll want your content to cluster around. Also related: “gravel bike,” “hybrid,” “commuter bike,” “frame geometry,” “bike fit.”

How to Measure Your Inseam

To size your frame accurately, you need your inseam — the inside leg length from crotch to floor. Here’s how:

1. Stand close to a wall with your feet ~15–20 cm (6–8 in) apart.
2. Place a hardcover book between your legs, pushed snug to the crotch (spine acting like a saddle).
3. Mark where the book’s spine touches the wall.
4. Measure from that mark to the floor — that’s your inseam (in cm or inches).

Note: many simpler formulas just use your height, but that ignores leg–torso ratio. Legs can be long or short relative to your height, so inseam-based sizing is more precise.

Frame Size Formulas & Guidelines

Below are the common formulas used. Use them as a starting point — always verify with the manufacturer’s sizing and geometry charts.

• City / Trekking / Hybrid bikes
   Frame size ≈ inseam (cm) × 0.64
• Road / Gravel bikes (based on the Guimard formula)
   Frame size ≈ inseam (cm) × 0.67
• Mountain bikes
   Mountain frame size ≈ (road frame size) – 11 cm
   —or subtract ~10–12 cm from the road equivalent

So, for example:
If your inseam is 81 cm → city frame ≈ 81 × 0.64 = 51.84 cm
Road frame ≈ 81 × 0.67 = 54.27 cm
Mountain ≈ 54.27 − 11 = ~43 cm (or check equivalent small/medium frame size)

These formulas give a range rather than a single “perfect” size. Always check with brand geometry and, if possible, test ride.

Bike Size Chart (Mountain and Road)

Here’s a rough chart to guide you. Because different brands measure differently, treat this as guidance, not gospel.

Inseam to Bike Frame Chart

Inseam (inches)	Inseam (cm)	Mountain Frame	Road Frame (cm)
25 – 25.5	64 – 65	12 – 13 in	43 – 44 cm
26 – 27	66 – 69	13 – 14 in	44 – 47 cm
27.5 – 28	70 – 72	14 – 15 in	47 – 49 cm
29 – 30	74 – 76	15 – 16 in	49 – 51 cm
30.5 – 31.5	77 – 80	16 – 17 in	51 – 54 cm
32 – 32.5	81 – 83	17 – 18 in	54 – 56 cm
33 – 34	84 – 86.5	18 – 19 in	56 – 59 cm
34.5 – 35.5	87 – 90	19 – 20 in	59 – 61 cm
36 – 37	91 – 94	20 – 21 in	61 – 63 cm
37.5 – 38.5	95 – 98	21 – 22 in	63 – 66 cm

Kids’ Bike Wheel Size by Inseam

Inseam (in)	Inseam (cm)	Wheel Size
14 – 17	35 – 43	12″
16 – 20	40 – 50	14″
18 – 22	45 – 55	16″
20 – 24	50 – 60	18″
22 – 25	55 – 63	20″
24 – 28	60 – 72	24″
28+	72+	26″

Again: these are guidelines. Use manufacturer sizing charts and, if possible, a test ride to confirm.

Bike Fit Tips & Adjustments

Once you pick a frame size, you still must fine-tune the fit. A frame is just the starting point — the aim is that the bike “fits you,” not the other way around.

Here are key adjustments:

• Seat height & position: When your pedal is at the lowest point, your leg should be nearly straight with a small bend at the knee.
• Reach & stem length/angle: Adjust to avoid overstretching or cramping.
• Handlebar height / spacers / rise: You might need to raise or lower handlebars.
• Saddle fore/aft position: Slide forward or backward to adjust knee-over-pedal alignment.
• Crank length / pedal stack / cleats: These also impact your comfort and power delivery.

Small changes—like a different stem length or moving spacers—can make a big difference. Don’t discard a frame just because it’s not perfect out of the box.

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What Size Frame Do I Need If I’m 183 cm Tall?

Let’s work through an example:

• Height: 183 cm
• Typical inseam estimate: ~47% of height → ~86 cm
• City/Trekking size: 86 × 0.64 = 55.0 cm
• Road size: 86 × 0.67 = 57.6 cm
• Mountain size: ~57.6 – 11 = 46.6 cm

So for a city bike, ~55 cm; for a road bike, ~57–58 cm; for a mountain bike, ~46–48 cm. But again — check the brand’s geometry and, if possible, test ride.

Final Thoughts & Caution

• Use the bike size calculator as a guide, not an absolute rule.
• Always check the manufacturer’s geometry charts.
• A professional bike fitting is ideal, especially for serious riders.
• Many parts (stem, handlebars, seatpost) are adjustable or upgradable—so don’t discard a “nearly right” frame prematurely.
• If buying from an online or uncertain source, double-check sizing and return policy.

FAQs

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