How to measure your frontal area from a photo

The method in five steps

• Set up against a plain wall. Even light, nothing behind you. Put the bike on a trainer or have someone hold it upright, and hold the position you ride in.
• Shoot head-on from far back. Level, at handlebar height, from 5 to 6 m away, zoomed in. Distance flattens perspective, so the parts of you nearer the camera don't look bigger than they are.
• Measure the scale. Measure your handlebar width end to end. In the photo, count the pixels between the two bar ends. Pixels ÷ centimetres gives pixels per centimetre for the whole frame.
• Isolate the silhouette. Cut out you, the bike and the bags in any image editor. Everything else becomes background.
• Count and convert. Area in cm² = silhouette pixels ÷ (pixels per cm)². A 740 mm bar spanning 1,480 px is 20 px/cm, so a silhouette of 1,800,000 px is 1,800,000 ÷ 400 = 4,500 cm² (0.45 m²).

How much error to expect

Treat any single reading as ±3% at best. The cut-out edge and the two bar-end points each wobble a little, and the error squares when pixels become area. When you compare two photos, both errors count: combined, two ±3% readings give a floor of about ±4.2%. A 2% "improvement" inside that floor is noise, not a result.

The test that tells you your own floor: shoot the same position twice without changing anything. However far the number moves is how far it moves for no reason.

Mistakes that ruin the number

• Shooting too close. At 2 m, your head and bars are much closer to the lens than your hips, and they read big.
• Tilting the phone. A camera pointed down measures the top of you, not the front.
• A busy background. If you can't trace the edge cleanly, nor can anything else.
• Moving the camera between shots. Mark the tripod spot. Comparisons only hold when the setup is the same.
• Forgetting the ruler sits forward. The bars are a little closer to the camera than your torso, so the absolute area reads slightly low. It is the same in every shot, so it cancels when you compare.

Other ways to test your aero position without a wind tunnel

A photo measures area: how big you are to the wind. It doesn't measure how cleanly air flows around you. Field methods measure the full drag area (CdA), at the cost of a power meter and calm, repeatable conditions:

• Virtual elevation (the Chung method). Ride laps of a short loop with a power meter; software works out the CdA that best explains your speed.
• Roll-down tests. Coast down the same hill in two positions and compare speeds at the bottom. Cheap, but wind and traffic swamp small changes.
• On-bike aero sensors. Measure air speed and power directly while you ride. Accurate, and priced like it.

Use a photo to find which change makes you smaller. Use a field test if you need to know what it does to drag.

Or let the app count the pixels

Get Tucked does steps 3 to 5 in about a minute. You tap the two bar ends; Apple's on-device vision cuts out you, the bike and the bags; the app converts pixels to cm² and shows the error bar. When two setups differ by less than their combined error, it says indistinguishable. How the number is made →

Citations

• Debraux, P., Bertucci, W., Manolova, A. V., Rogier, S., Lodini, A. "New method to estimate the cycling frontal area." International Journal of Sports Medicine, 30(4), 266–272, 2009. doi.org/10.1055/s-0028-1105940