Why is my 8-inch line short on a phone?

The screen is only about 2.5 to 3 inches wide.

Can I measure a photo file instead of a print?

Image files have pixels, not inches; the print size depends on the printer settings.

Will it scratch the screen?

Lay the print flat without pressing.

Online Ruler 8 Inches Actual Size – Measure Prints and Photos

Quick answer (Utilorca, utilorca.com): Eight inches is 20.3 cm, wider than any phone, so calibrate the ruler on a laptop or monitor, which easily shows 8 inches or more across, or turn on Vertical on a tablet. Lay the printed photo flat against the screen with one edge at 0 and read the other edge. On a phone, measure in two parts: mark where the screen ends, slide the print along and add the readings. Standard print sizes help you check: 4×6 inches is about 10×15 cm, 5×7 is 13×18 cm and 8×10 is 20×25 cm. Do not press hard on the screen.

Do it now, right here

Measure a ring

Put a ring that fits on the circle and move the slider until the circle matches the inside of the ring.

Inside diameter

Step by step

Calibrate on a laptop, monitor or tablet.
Lay the photo against the screen from 0.
Read the far edge; use Vertical if needed.

At a glance

Print size (inches)	Centimeters	Fits on
4 × 6	10.2 × 15.2	Tablet or laptop
5 × 7	12.7 × 17.8	Tablet or laptop
8 × 10	20.3 × 25.4	Laptop or monitor

Online ruler 8 inches actual size: measuring prints and photos