Drainage

Pipe Drop Calculator

You know how long the run is and what slope you want. This calculator tells you how much lower the downstream end has to sit, in inches, feet and metric units, so you can set your string line before you start digging.

Your measurements

Results update as you type. Every unit dropdown converts for you.

Pipe length

Slope entered as

Slope

Total drop

9.60inches

Total drop in feet: 0.800 ft
Total drop in centimeters: 24.38 cm
Total drop in meters: 0.244 m
Drop per foot: 0.240 in
Slope percentage: 2.00%

Show the calculationOpen

Slope as a decimal2.00% ÷ 100=0.0200
Total drop40.00 ft × 0.0200=0.800 ft
Convert to inches0.800 ft × 12=9.60 in

Reading your result

The headline figure is the height difference to build between the two ends — how much lower the outlet sits than the inlet. Set your string line at the inlet invert and measure this distance down at the far end.

The drop-per-foot figure is the one to use for intermediate stakes. Marking grade every few feet as you go beats discovering at the end that the middle of the run sagged.

Add this drop to your starting trench depth to see how deep the far end has to be dug. That number decides whether the run is feasible before you break ground.

How this calculator works

This is the pipe slope calculation run backwards. Instead of measuring a drop and deriving the slope, you choose the slope and derive the drop.

Slope entered as a percentage is divided by 100 to become a decimal. Slope entered as inches per foot is divided by 12 instead. Either way you end up with the same kind of number, and the total drop is that decimal multiplied by the run.

The result is the height difference between the two ends of the run — not a per-joint figure and not a depth of excavation. Trench depth is that drop plus whatever cover you are working to at the upstream end.

Metric outputs are provided for anyone working from drawings in millimeters or centimeters, using the exact 0.3048 metre-per-foot conversion.

The formula

Slope from percent: percentage ÷ 100
Slope from inches per foot: inches per foot ÷ 12
Total drop: pipe length × slope
Drop in inches: total drop in feet × 12

Worked example

A 40 ft run set at a 2% slope.

01Slope as a decimal2 ÷ 100=0.02
02Total drop40 ft × 0.02=0.8 ft
03In inches0.8 × 12=9.6 in
04In centimeters0.8 ft × 30.48=24.4 cm
05Drop per foot0.02 × 12=0.24 in

The outlet end sits 9.6 inches lower than the inlet end — call it 9⅝ inches on a tape.

How to take the measurements

Measure the run horizontally between the two fixed points the pipe has to connect, not along the ground surface.
Set your string line at the upstream invert height, then measure the calculated drop down from that line at the far end.
For long runs, mark intermediate stakes using the drop-per-foot figure so you can check grade as you go rather than only at the ends.
If the downstream connection height is already fixed, work out the drop first and check that the upstream end still ends up at a workable depth.
Add the drop to your upstream trench depth to see how deep the far end has to be dug.

Practical considerations

Work out the drop before you dig, not after. If the downstream connection point is already fixed by an existing pipe or a daylight outlet, the drop tells you how high the inlet has to start.
Where the required drop would put the outlet below your discharge point, the run is not viable at that grade. Reducing slope, shortening the run, or relocating the outlet are the options.
Remember cover. A deep outlet is fine; an inlet so shallow that the pipe sits above frost depth or under a driveway with no cover is not.
For long runs, a laser or rotary level pays for itself. String lines sag measurably over 50 feet, and the sag is the same order of magnitude as the drop you are trying to set.

Assumptions and limitations

Drop is pure geometry from the length and slope you supply. Whether that slope is appropriate for your pipe and application is a separate question.
The figure describes the two end points only. It says nothing about keeping the grade consistent in between, which is what actually determines whether the pipe drains.
Trench depth, cover, bedding and frost depth are not part of this calculation.

Frequently asked questions

Is drop the same as trench depth?

No. Drop is the height difference between the two ends of the pipe. Trench depth at the far end is that drop plus the depth you started with at the upstream end.

Which should I enter — percent or inches per foot?

Whichever your plan or your habit uses. They describe the same thing, and the calculator shows both in the results.

Does pipe diameter change the drop?

Not in this calculation. Diameter affects capacity and the slope you might choose, but the drop for a given length and slope is pure geometry.

How precise does the drop need to be?

Aim to be within a fraction of an inch over a typical residential run. Consistency matters more than perfection: a run with no bellies at a slightly shallower grade behaves better than one that hits the target on average but dips in the middle.

Is the drop the same as how deep I need to dig?

No. Drop is the height difference between the two ends of the pipe. Trench depth at the far end is your starting depth plus this drop.

What if I cannot get the full drop over my run?

Then the slope has to come down, the run has to get shorter, or the end points have to move. Accepting a shallower grade is a judgement call that depends on the pipe and application, and on local requirements.