Bike Fit Calculator: Frame Size, Saddle Height, Stem & What to Adjust (Free)

"Will this bike fit me?" is the question behind most frame-size charts, and the chart usually answers it with a single number: your height. But size labels aren't standardised. A size M from one brand can be longer and lower than an L from another, and the height chart assumes average proportions.

Stamptivity's Bike Fit tool answers the question from the numbers that actually decide your position: your measurements, and the bike's stack, reach and tube angles. It's free and runs in your browser.

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What It Tells You

For each bike you add, the fit result shows:

• How it fits: standover clearance, how much seatpost you'll have, where your hands end up with the stock cockpit, and how big a stem or spacer change you'd need. Each factor is shown on its own. There's no hidden overall score.
• A recommended starting setup: saddle height, saddle setback, stem length and angle, spacer height and, for drop bars, a handlebar width, each as a range.
• What to adjust: if you enter the setup you ride now, concrete changes such as "raise the saddle about 15 mm" or "try a 90 mm +6° stem with 15 mm of spacers", or confirmation that a value is already in range.

Everything comes with a confidence level. Saddle height from your inseam is fairly reliable. Where your hands should be depends mostly on how upright you like to sit, so that part is always shown as a range. Measuring your torso and arm raises the confidence from Low to Medium, which is as high as the tool goes from body measurements alone.

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How to Measure Yourself

Height and inseam are enough to start. Torso and arm length narrow the result. Measure three times, ideally with a helper; the tool's 3× button turns three readings into a range.

Height: barefoot, back against a wall.

Inseam: barefoot, feet 15–20 cm apart. Press a thick book firmly up between your legs, level like a saddle, and measure floor to the top of the book.

Torso: from the top of the pubic bone (the top of the book in the inseam method) to the C7 vertebra, the bump at the base of the neck. Don't start from the hip bone (iliac crest). It reads roughly 8–12 cm short, and the tool flags a torso that doesn't match your height and inseam.

Arm: from the acromion (the bony tip of the shoulder) to the centre of the wrist, arm relaxed.

Shoulder width: acromion to acromion, across the back. It sets the suggested drop-bar width.

If you're unsure of a measurement, enter a range or mark it low confidence. Uncertainty is carried through every result instead of being hidden.

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Reading a Geometry Chart

Search for "[bike model] geometry" to find the manufacturer's chart. You need four numbers:

• Stack: vertical distance from the bottom bracket to the top of the head tube.
• Reach: horizontal distance between the same two points.
• Seat tube angle and head tube angle.

Standover and seat tube length add the standover and seatpost checks. Head tube, wheelbase, chainstay, BB drop and fork numbers make the side-view drawing accurate and add handling notes. The tool also cross-checks the chart: if stack, reach, seat angle and top tube don't agree, one of them is probably mistyped.

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Why Stack and Reach Aren't Your Hand Position

Your hands sit on top of the frame numbers: headset cap, spacers, stem, and the bar's own rise and reach. Because the steerer leans back and stems are angled, each part moves your hands in two directions at once. On a typical 73° head tube with a 90 mm −6° stem, the tool's own model gives:

Change	Hands move forward	Hands move up
+10 mm spacers	−3 mm	+10 mm
+10 mm stem length	+10 mm	+2 mm
Flip stem from −6° to +6°	−6 mm	+18 mm
+20 mm frame stack	0 mm	+20 mm

The handlebar matters too. On a drop bar, the hoods sit on the front of the bend, so the bar's reach, drop and rotation all move them. Rotating the bar up a few degrees raises the hoods and pulls them slightly back. The tool draws the bar's actual profile and places the hoods on it.

That's why the tool fits a stem and spacers to each frame instead of comparing raw stack and reach. It also shows when a frame only fits with unusual parts, like a 50 mm stem or a 40 mm spacer stack. That usually means the size is at the edge for you.

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Checking a Bike You Already Ride

Add the bike, then enter its current setup:

• Saddle height: BB centre to the top of the saddle, along the seat tube line.
• Saddle setback: hang a plumb line from the saddle nose and measure horizontally back to the BB centre.
• Stem: length and angle are printed on it. Note whether it's flipped (+6° instead of −6°).
• Spacers: the stack of spacers under the stem.

The fit result then compares each one with the recommended range and tells you what to change, one step at a time. Anything you leave empty turns into a "set it to about…" instruction instead of a guess.

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Choosing Between Sizes

Add the same model in two sizes, or two different bikes, and a compare table appears with every factor side by side.

The tool's built-in example shows why this matters. A 174 cm rider rides a road bike in size S and is considering the M. The manufacturer's height chart puts them in the M (168–176 cm), with the S ending at 170 cm.

The fit result is less one-sided. Both sizes can be set up for this rider with ordinary parts, but they differ in two ways worth knowing before buying:

• Standover: 63 mm of clearance on the S, only 26 mm on the M.
• Cockpit: the S is already close to the target position with the stem and spacers fitted; the M would want a 90 mm +6° stem with 15 mm of spacers to get there.

So the height chart's "M" is defensible, but the S is the safer frame for this rider, and the M is a change of cockpit rather than a drop-in.

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Adjust & Compare

Adjust & Compare lets you try changes on a bike, such as a shorter stem, more spacers, a riser bar or a rotated handlebar, and see the result in millimetres and on a same-scale side view. By default it compares with where your measurements put you.

If you own a bike that already feels right, add it and choose it under Compare with. That compares the bike you're adjusting against it, and two shortcuts appear:

• Copy setup puts the other bike's stem, spacers, handlebar and saddle position on this one, so you can see how much the frame alone changes things.
• Match keeps this bike's handlebar and finds the stem and spacers that put your hands where they are on the other bike.

A comfortable bike is the strongest evidence you have of the position that works for you, especially once its saddle height, setback and spacers are measured.

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Using Stamptivity Bike Fit

1. Open Stamptivity Bike Fit, or click Load example to explore the case above with a rider and two bikes already filled in.
2. Enter your height and inseam, plus torso and arm length if you have them.
3. Add a bike from its geometry chart. The quickest way is Paste geometry chart: copy the whole table from the manufacturer's site and paste it, and every size is read at once. Load example bikes has a few manufacturer charts already typed in, and Add by hand opens an empty bike's form. Then add the bike's current setup if you ride it.
4. Read the fit result: how it fits, the recommended starting setup, and what to adjust.
5. Optionally add more bikes to compare them side by side.
6. Try changes in Adjust & Compare, against your measurements or another bike, and open the report for a printable summary.

Everything stays in your browser. Copy link packs your measurements and bikes into the address itself, so you can send a fit to someone else or to your own phone; Export JSON saves a full backup, scenarios included. A link carries your measurements, so share it only with people you want to see them.

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Flat Bars and Mountain Bikes

Switch a bike's handlebar type to flat bar and the hand reference becomes your grips, with backsweep treated as a negative bar reach. Saddle height and setback work the same on any bike. Mountain bikes also depend on suspension sag and a more dynamic riding position, which a static geometry model can't capture.

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Limitations

This is a geometry-based starting point, not a professional bike fit. It can't see your flexibility, pedalling mechanics, cleat position or any pain. Hand position on the hoods also depends on lever model and hood angle, which is why it's shown as a range. If something hurts, see a fitter before making big changes.

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Tips

• Measure carefully once rather than guessing twice. Inseam drives saddle height, and torso and arm length drive the handlebar target.
• Change one thing at a time on the bike and ride a few times before the next change.
• Watch for "uncommon parts". A frame that only fits with a very short or long stem is telling you something about its size.
• Compare against a bike that already feels right whenever you have one.
• Coming from another calculator? See how this compares with the Competitive Cyclist fit calculator, or when a geometry calculator beats an AI video fit.
• Once your bike is dialled in, use Stamptivity Stamp or Overlay to turn your next ride into a shareable image or video.