How do you measure a slope?
Point the top edge of the phone up the slope, lay it flat, and tap Measure Slope.
The solid line on the gauge moves to match the surface. The dashed line stays horizontal, so you can compare the two.
If you turn the phone to face downhill, the same slope reads as a negative number.
On gravel or rough ground, rest the phone on a batten or another straight edge. Otherwise you measure one stone rather than the slope of the ground.
Which unit should you use?
Percent is used for roads and cycling. Ratios are used for anything that carries water. Soil pipes run at 1:40 to 1:80, patios at about 1:80, and shower trays at about 1:50.
Ramps are usually between 1:12 and 1:20. Degrees are used for solar panels, joinery and machining. Tap a unit to move it into the large display.
Three values are useful to remember. An angle of 1° is 1.7%, or 1:57, or 17 mm per metre. An angle of 10° is 17.6%.
An angle of 45° is 100%, not vertical. Roofers use rise over twelve instead, which is on the Roof Pitch Calculator.