Gutter Angle Calculator
Start from a fall angle in degrees and get the slope and drop you can actually mark out — inches per foot, millimetres per metre, percent and ratio.
Enter a fall angle and the run — get the drop to mark plus the slope per foot and metre.
What is the Gutter Angle Calculator?
The Gutter Angle Calculator exists because gutter slope is most naturally measured as a tiny angle in degrees, yet most installation instructions quote drop in inches per foot or millimetres per metre. Bridging that gap by hand requires inverse-tangent arithmetic that is easy to get wrong at a fraction of a degree. This tool does the conversion instantly — enter any angle and read the equivalent slope, drop and ratio, or enter a slope and read the matching angle — so you can move straight from the drawing to marking the fascia without intermediate calculation errors.
Gutter Angle Calculator Formula
The relationship between a gutter angle and its slope is the tangent function:
Slope (rise ÷ run) = tan(angle°)
Reversing it to find the angle from a known slope uses the inverse tangent:
Angle = arctan(slope) × (180 ÷ π)
For example, a 0.5° gutter has a slope of tan(0.5°) ≈ 0.00873, which converts to about 0.105 in/ft or 8.7 mm/m. A common 1:480 slope (0.0020833) gives arctan(0.0020833) ≈ 0.119°. Because gutter angles are well under 1°, the small-angle approximation (slope ≈ angle in radians) is nearly exact, but the full tangent formula is used here for precision.
The gutter angle calculator works the angle both ways. Leave the unit on degrees to enter a fall angle and read the equivalent slope and drop, or switch the unit to a ratio, percentage or inches-per-foot to convert the other direction. Because the maths is rise ÷ run = tan(angle), every form stays perfectly in step, and the preview tilts so a sub-degree slope is visible at a glance.
Turn degrees into a mark
If you already know the slope and just want the degrees, the gutter slope angle page is set up for that direction. For full run, drop and outlet planning, open the main calculator.
Questions answered
How do I calculate a gutter angle?
What angle is a 1 in 100 gutter fall?
How many degrees is a quarter inch per foot?
Should I set my gutter by angle or by drop?
What angle finder do I need for gutters?
The complete gutter tool suite
Every gutter question has its own purpose-built calculator — fall, slope, pitch, angle, flow and outlet spread, for the US, UK, Australia and New Zealand.
Need the full gutter slope toolkit?
Switch between drop, slope check, max run and downspout planning on the main calculator — with the same live diagram.