How We Calculate: The Numbers Behind Every Tool, and Where They Come From
Every calculator on this site runs on published formulas and openly stated constants — the 0.623 gallons per square foot per inch of rain, the 40 percent void space of clean stone, Manning's equation for pipe capacity, NOAA Atlas 14 rainfall — with a margin built in on the cautious side. This page lists each one, its source, and the checks the logic passes before it ships.
What the tools are, and are not
The calculators on this site are planning tools. They turn a homeowner's measurements into the same quantities a contractor or designer would start from — gallons, GPM, cubic yards, square feet, planning cost ranges — using the methods those professionals use, simplified only where a homeowner cannot supply the input. They are deliberately biased toward caution: runoff factors on the high side, pipe capacities de-rated, grate capacities de-rated for leaves, drain-down windows enforced.
They are not designs, and they do not replace anyone standing on the property. Every tool ends with a "what this doesn't tell you" section for that reason, and every cost figure is a planning range, never a quote. The editorial policy covers the site's independence; this page covers the arithmetic.
Constants every tool shares
| Constant | Value | Used by | Source |
|---|---|---|---|
| Gallons per square foot per inch of rain | 0.623 | Roof runoff, dry well, rain garden | Physical: 1 in × 1 sq ft = 231 cu in ÷ 231 cu in/gal × 1.44 |
| Gallons per cubic foot | 7.48 | Dry well, rain garden | Physical constant |
| Roof runoff factor | 0.95 | Roof runoff, French drain sizing, catch basin, rain garden | Rational-method C for roofs, 0.85–0.95; the high end is used so water is never under-estimated |
| Paving runoff factor | 0.90 | French drain sizing, catch basin, rain garden, dry well | Rational-method C for concrete and asphalt, 0.80–0.95 |
| Lawn runoff factor | 0.15 sandy · 0.25 loam · 0.35 clay | French drain sizing, catch basin, rain garden | Rational-method C for lawns by soil group, flat to moderate slope |
| Square feet per acre | 43,560 | Rational-method tools | Definition |
| Cubic feet per second to GPM | 448.83 | Rational-method tools | Definition |
| Void space of clean #57 stone | 40% | Dry well, gravel sizes | ASCE MOP 77 and supplier data, 35–45% for clean angular stone |
| Weight of #57 stone | 1.4 tons per cubic yard | Every stone quantity | Supplier weight tickets, NCDOT unit weights, 1.35–1.45 |
| Bagged stone | 0.5 cu ft per 50-lb bag | Gravel calculator | Retail packaging |
| Loose-soil swell | 25% | Gravel calculator, retaining wall | Standard excavation swell factor for topsoil and clay |
Rainfall
| Figure | Value | Source |
|---|---|---|
| Storm depth presets | 1, 2 and 3 inches | Typical soaking, heavy, and downpour days for the eastern US; the 2-year 24-hour NC depth is about 3.5 in |
| Regional peak intensities | 3, 4, 6 and 8 in/hr | Rounded from NOAA Atlas 14 10-year 5-minute values, set a little under the point value because a residential catchment never delivers its whole peak at once |
| City lookup | 2-year and 10-year 24-hour depths for 59 cities | NOAA Atlas 14 partial-duration point estimates at city-centre coordinates, retrieved September 2026; full NC table on the NC rainfall page |
Soil
| Figure | Value | Source |
|---|---|---|
| Sandy preset | 2.0 in/hr | Low end of the published range for sand, so sandy yards are not oversold |
| Loam preset | 0.5 in/hr | Top of the loam range — a healthy, uncompacted lawn |
| Clay preset | 0.1 in/hr | Top of the clay-loam range; true clay is slower |
| Interpretation bands | free > 4 · normal 1–4 · slow 0.5–1 · poor < 0.5 in/hr | Site thresholds, consistent with rain garden and dry well drain-down limits; full table on soil infiltration rates |
| Rain garden drain-down limit | 24 h good, 48 h maximum | Extension-office rain garden guidance (UConn NEMO, Alabama ACES, Wisconsin DNR) |
| Dry well drain-down limit | 24 h good, 72 h maximum | ASCE MOP 77 and state stormwater manuals (NH, PA) |
Pipe and grate capacity
| Figure | Value | Source |
|---|---|---|
| Pipe capacity | Manning's equation, full flow, R = D/4 | Standard open-channel hydraulics |
| Manning's n | 0.013 smooth PVC · 0.020 corrugated | Published roughness coefficients for PVC/SDR-35 and corrugated HDPE |
| Usable share of full-pipe capacity | 75% | Margin for fittings, grit and a perforated pipe in gravel never running full |
| Minimum workable grade | 1% (⅛ in per ft) | Plumbing-code minimum for drain pipe; below it sediment settles |
| Grate capacities | 28 · 96 · 155 · 264 · 500 GPM for 6, 9, 12, 18, 24-inch grates | Manufacturer spec sheets at ½ in of ponding, low end of each published range |
| Usable share of grate capacity | 60% | Allowance for leaves and grit after the first season |
| Downspout capacity rule | 1 sq in of downspout per 100 sq ft of roof at 4 in/hr | SMACNA Architectural Sheet Metal Manual sizing rule |
Retaining walls
| Figure | Value | Source |
|---|---|---|
| Buried course | One full course, or 10% of exposed height | Segmental block manufacturer installation guides |
| Base | 6 in compacted stone, block depth + 12 in wide | Manufacturer guides and the site's backfill specification |
| Drainage zone | 12 in of clear stone, full height, 6 in topsoil cap | Same; detail on retaining wall drainage |
| Geogrid threshold | Above 3 ft exposed | Typical manufacturer charts; the tool's quantity is a planning figure only |
| Engineer and permit threshold | Above 4 ft | Common local code threshold; some jurisdictions measure from the base |
| Block presets | 4×12×7 · 6×16×12 · 8×18×12 in · 6×6×8 ft timber | Common retail and manufacturer units and published weights |
Cost ranges
Every dollar figure is a planning range drawn from the site's own cost pages, which in turn draw on published aggregator ranges (Angi, HomeGuide, LawnStarter, Homewyse) cross-checked against Southeast contractor pricing and material prices. The same range is used wherever a fix appears, so the cost calculator, the hiring guide and each solution page agree. Site factors — no machine access, clay or rock, restoration — are applied as multipliers or allowances stated on the tool. None of it is a quote, and the tools say so.
How the logic is tested
Every calculator's arithmetic lives in a plain function separate from its screen, and each function has an automated test suite that runs before any change is published — worked examples with known answers (a 500 sq ft roof in a 1-inch storm makes about 311 gallons; 83 GPM at 1 percent needs a 6-inch PVC pipe), the thresholds where a verdict flips, and the edge cases where inputs are missing. At the time of writing the suites hold 130-odd checks across fourteen tools. When a constant on this page changes, the tests that depend on it fail until the change is reviewed.
Reporting a problem
If a result looks wrong, the fastest fix is a worked example: the inputs you entered, the answer you got, and the answer you expected with its source, sent through the about page. Corrections are made in the logic and its tests, and this page is updated with the new figure and source.
By Under Your Home Team · Last reviewed 2026-09-07