Fit the ball outline
Line the amber outline up with the edge of the ball. Drag the centre dot to move it and the edge dot to resize it.
Lens distortion
Mark the grip
- TThumb hole
- MMiddle-finger hole
- RRing-finger hole
Trace the first oil ring
Tap along the middle of the oil ring nearest the holes. Spread 8–15 points over as much of the ring as you can see. Tap a point to select it, drag to move it.
Turn on Enhance above the photo if the ring is hard to see. Red points sit off the fitted ring; check or delete them.
This is a synthetic demo ball with a known answer. Measure your own ball:
How the measurement works
Camera. The photo is treated as a pinhole camera. The focal length comes from the photo's EXIF data (35 mm equivalent) or from your lens choice, converted to pixels using the sensor diagonal.
Ball. Seen through a lens, a sphere's outline is where a cone of rays from the lens just grazes it. The outline's angular radius α gives the distance to the ball's centre, D = r / sin α. Close-up photos see less than half the ball, and the edges are squashed; the model accounts for both.
Marks. Every tap is turned into a viewing ray and intersected with the 3D ball, giving a true point on its surface.
Track. The first oil ring is every point that touched the lane while the ball turned about its release axis, so it is a circle around that axis. A plane is fitted through the ring points; its normal through the ball's centre is the axis, and the end on your bowling-hand side is the PAP. With zero tilt the ring is 90° (6¾ in) from the PAP; tilt moves it further away, and the gap gives the tilt estimate.
Measure. Over is the distance along the horizontal midline from the centre of grip; Up is the distance from the midline to the PAP, square to it. Both are arc lengths on the ball's surface, the way a layout tape reads them.
No-thumb grips. The centre of grip is the middle of the bridge between the finger holes, as USBC defines it for thumbless balls. The horizontal midline runs through both finger holes and the vertical axis line crosses it square at the bridge. With a right hand the middle finger sits left of the ring finger in layout view (mirrored for a left hand), which tells the app which way is up.
Uncertainty. The ± figure re-runs the solution 160 times with every mark, the outline and the focal length jiggled by realistic amounts.