Fall and Gradient Calculator

A gradient is written three different ways and people mix them up on the same job. “One in eighty” is a ratio, “1.25%” is the same thing, and “12.5mm per metre” is the same thing again in the units you actually mark out with. Get them confused and the patio drains towards the house.

What the fall is for

How you want to give it

Length of the run in metres

End to end.

One in how many

A bigger number is a gentler slope.

50mm of fall

1 in 80, which is 1.25%, 12.5mm a metre, or 0.72 degrees. Over 4m the far end sits 50mm lower.

Along the run	Drop from the high end
0m	0mm
1m	12.5mm
2m	25mm
3m	37.5mm
4m	50mm

✓ The ratio, the percentage and the millimetres agree — 1 in 80 is 1.25%, which is 12.5mm a metre
✓ The total fall is the fall per metre times the run — 50mm over 4m — the far end sits that much lower
✓ The angle converts back to the fall — 0.72 degrees, which is the same slope written a fourth way
✓ Every mark along the run is on the same slope — 5 marks, each 12.5mm below the one before
✓ Steep enough for the job — 1 in 80 against a usual minimum of 1 in 80 for patio or paving — remember a bigger number is a gentler slope
✓ Not steeper than the quoted maximum — the usual maximum is 1 in 40

The figures quoted for each job are the commonly used ones. Drainage and access work is regulated, and this is arithmetic rather than approval.

What this generator does

Reduces whichever form you give to millimetres of fall per metre, converts that into the other three, and lists the drop at each metre along the run.

How to use this tool

Choose what the fall is for — paving, a pipe, a gutter, a ramp.
Say how long the run is.
Give the gradient in whichever form you have it.
Mark the drop at each metre along the run.

Understanding the controls

What the fall is for: Paving, paths, soil and waste pipes, gutters, ramps and drives all have their own commonly quoted minimum and maximum falls, which the answer is compared against.
How you want to give it: As a ratio, a percentage, millimetres per metre, or simply the total drop you have measured over the run. Any of them describes the same slope.
Length of the run: End to end in metres. This is what turns a gradient into a total fall, and what the marks are spread along.
The gradient itself: Whichever form you chose. A bigger ratio number is a gentler slope, which is the part that catches people out — 1 in 200 is much shallower than 1 in 40.

Common use cases

Laying a patio to fall away from the house
Setting a soil pipe to the right gradient without guessing
Working out the fall on a gutter to its outlet
Checking whether a ramp is within the usual limit
Converting a drawing's percentage into millimetres you can mark

How this generator works

Everything reduces to one number: millimetres of fall per metre. From it the ratio is a thousand divided by the fall, the percentage is the fall over ten, and the angle is its arctangent. The check runs all of those back: the three forms must describe the same slope, the angle must convert back to the fall, and the total fall must be the fall per metre times the run. The marks along the run are each checked to sit on the same straight line, so a string line pulled to them is straight rather than approximately straight.

Randomness and fairness

Nothing is random. The same slope always converts the same way.

For how randomness is produced across the whole site, see how Generate Random works.

Limitations and good to know

The figures quoted for each job are the commonly used ones; drainage and access work is regulated and this is arithmetic rather than approval.
Pipe gradients that are too steep leave solids behind, which is why a maximum is given as well as a minimum.
Ground rarely falls in a straight line; the marks assume a level datum to measure down from.
Ramps have rules about landings and handrails that a gradient alone does not cover.

Privacy and your data

The conversion runs in your browser. No measurements are transmitted or stored.