What this calculator gives you
A gravity drain or sewer run only works if the invert drops a consistent amount across its length. This tool takes that run length in feet and a slope rate in inches per foot and returns the total fall in both inches and feet, the equivalent slope percent, and the grade. Give it a starting invert elevation as well and it returns the invert at the downstream end. The slope dropdown carries presets of 1/8, 1/4, 1/2, and 1 inch per foot, plus a Custom option for any rate you type yourself.
The math behind each row
Total fall in inches is pipe length × slope rate. Divide by 12 for total fall in feet. Slope percent is (slope rate ÷ 12) × 100, and grade is simply slope rate ÷ 12, the ratio of vertical drop to horizontal run. Ending invert is starting invert − total fall in feet, which is why that row only appears once you supply a starting elevation. Everything on the page derives from those five lines; there is no hidden adjustment.
Worked example at the default settings
The tool loads with a 100-foot run at 1/4 inch per foot. That is 100 × 0.25 = 25.00 inches of fall, or 25 ÷ 12 = 2.083 feet. Slope percent works out to 0.25 ÷ 12 × 100 = 2.083%, and grade to 0.02083. Type 100.00 into the starting invert field and the ending invert reads 97.917 feet. Swap the preset and the same 100-foot run gives 12.5 inches of fall at 1/8 inch per foot (1.042%), or 100 inches at 1 inch per foot (8.333%) — which is why the preset you pick changes trench depth at the far end by more than seven feet.
Where runs go wrong
The most common error is a unit mismatch: drawings that call out slope as a percent or a ratio have to be converted to inches per foot before they go in the rate field. The second is measuring the run along the ground instead of as true horizontal distance, which inflates the length and the fall with it. The third is treating the calculated fall as the whole story when fittings, bends, and structures consume elevation of their own. Once the downstream invert is settled, the depth drives the dig, so it is worth running the numbers through the excavation calculator for spoil volume and truck loads before the machine shows up. For the surface work above the trench, the grading and slope calculator handles cut and fill, and on-site systems have their own sizing problem in the septic drainfield sizer.
Limitations
This is a single straight run at one uniform slope. The calculator does not size pipe, check capacity or velocity, consider pipe diameter or material, model manholes, cleanouts, or drop structures, or evaluate whether a slope is permitted. It also has no idea what elevation datum you are working in, so the ending invert is only as trustworthy as the benchmark you typed. The reference panel on the page summarizes minimum slopes for orientation only; requirements differ by pipe size, code edition, and local amendment, so confirm every run against your adopted plumbing or civil code and the engineer’s drawings. Use the copy button to paste the figures into a layout sheet, or the estimate button to push the run across as a linear-foot line item with the price left at zero for you to fill in.
Questions about the math
Why is there no ending invert in my results?
The starting invert elevation field is optional, and the ending invert row only renders once you type a number into it. Leave it blank and the calculator reports total fall, slope percent, and grade but nothing about elevations, because it has no datum to work from. Enter your benchmark elevation at the upstream end and the ending invert appears as starting invert minus total fall in feet.
How do I enter a slope that the drawings give as a percent?
Run the tool’s own conversion backwards. Slope percent = (rate in inches per foot ÷ 12) × 100, so rate in inches per foot = percent ÷ 100 × 12. A 1% slope is 0.12 inches per foot and a 2% slope is 0.24 inches per foot. Choose Custom in the slope dropdown and type that converted value into the inches-per-foot field.
What is the grade figure in the results table?
Grade is the slope rate divided by 12 — the dimensionless ratio of vertical drop to horizontal run, shown to five decimals. A 1/4 inch per foot slope is a grade of 0.02083. It matters because many pipe lasers and digital slope levels are set in grade or percent rather than in inches per foot, so the grade and slope percent rows are the ones you dial into the instrument.
Does this calculator check my slope against plumbing code?
No. It performs arithmetic on the length and slope you give it and nothing else. The reference panel beside the inputs is a summary for orientation, not an approval, and required minimum and maximum slopes vary by pipe size, by code edition, and by local amendment. Verify every run against the plumbing or civil code your jurisdiction has adopted and against the engineer’s drawings before you dig.