Drainage Slope & Drop Calculator
Check total fall for yard drains, patio runs, driveway channels, retaining-wall daylight runs, and simple pipe-fall checks without dragging a full material takeoff into a straightforward slope decision.
Use this page for a fast bridge number or sanity check, then continue into the related calculators or guides below when the decision needs more than a raw conversion.
Shared slope utility
Run length to total drop
Choose the closest preset, keep the default target slope, or switch to measured drop when you already have field grades.
French drains commonly target about 1% fall so water keeps moving without forcing the trench into an overly steep grade.
Related Tools
French Drain Calculator
Estimate gravel, pipe, fabric, and trench takeoff once the fall path checks out.
Calculate →Runoff Catchment Calculator
Confirm contributing area and branch choice before you size a drainage family around the slope result.
Calculate →Channel Drain Calculator
Move into channel sections, outlets, and concrete support once the hardscape fall path checks out.
Calculate →Catch Basin & Downspout Calculator
Switch to a basin and solid-pipe branch when roof leaders or yard inlets own the job.
Calculate →How the utility stays narrow
This page owns slope, total drop, ratio, and fit state. It does not take over French-drain pipe counts, gravel quantities, retaining-wall drainage packages, or patio material takeoffs. Once the fall path looks workable, move into the branch calculator that owns the actual shopping list.
Start with runoff when you are still diagnosing the source, French drain for trench-and-rock work, channel drain for hard-surface intercepts, and catch basin/downspout when roof leaders or inlets own the branch.
Core formulas
Drop = Run x (Slope % / 100)
Slope % = (Drop / Run) x 100
Quick reference: 1% fall equals about 0.12 in/ft or 10 mm/m. The preset table below keeps those equivalents attached to the specific use case instead of forcing one generic rule onto every surface.
Preset quick reference
| Preset | Target | Typical band | Equivalent |
|---|---|---|---|
| Patio / walkway away from structure A 2% fall is the common planning target for pavers and walkways that need to shed water away from a structure. | 2.0% | 1.0% to 3.0% | 0.24 in/ft or 20 mm/m |
| Walkway fall Walkways often use a gentler fall than patios, but they still need enough drop to move water off the surface. | 1.5% | 1.0% to 2.5% | 0.18 in/ft or 15 mm/m |
| Driveway drainage check Driveways usually need a clear drainage fall, but transitions and traction matter more than chasing a steeper slope. | 2.0% | 1.5% to 4.0% | 0.24 in/ft or 20 mm/m |
| French drain line French drains commonly target about 1% fall so water keeps moving without forcing the trench into an overly steep grade. | 1.0% | 0.5% to 2.0% | 0.12 in/ft or 10 mm/m |
| Channel drain run Channel drains usually need just enough fall to keep the trough and outlet moving toward discharge. | 0.5% | 0.3% to 1.0% | 0.06 in/ft or 5 mm/m |
| Downspout tie-in Short tie-ins can tolerate a bit more fall, but the service should still check that the run stays practical. | 2.0% | 1.0% to 4.0% | 0.24 in/ft or 20 mm/m |
| Retaining wall backdrain Backdrain runs usually need modest fall to daylight or to a collector without stealing room from the wall assembly. | 1.0% | 0.5% to 2.0% | 0.12 in/ft or 10 mm/m |
| Regrade / surface fall Regrade work should preserve a clear fall path while keeping the final grade realistic for the site. | 1.0% | 0.5% to 2.0% | 0.12 in/ft or 10 mm/m |