How We Calculate: The Numbers Behind Every Tool, and Where They Come From

Short answer

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, rainwater harvesting, 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
Average rainfall by zip	Monthly and annual 1991–2020 normals for every zip	NOAA NCEI gridded normals (nClimGrid, 1/24°) at the zip's Census 2020 internal point; nearest qualifying NCEI station normal (flags S, R or P, within 100 km) in Alaska, Hawaii and Puerto Rico. Method and open data on the rainfall by zip code page
Zip hand-off into the calculators	Dry well: 2-year 24-hour depth · rain garden overflow check: 2-year 24-hour depth · catch basin: 10-year 5-minute intensity · French drain sizing: 10-year 1-hour intensity	NOAA Atlas 14 values for the zip's county; each matches the duration the tool's own presets are built on
Rainwater collection efficiency	85% metal · 80% asphalt shingle · 75% tile, wood or gravel	Texas Water Development Board: most installers use 75–85% after first flush, evaporation and splash-out

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 (gutter size calculator)	1 sq in of downspout inlet drains 1,200 ÷ I sq ft of design area; design area = plan × 1.00–1.30 pitch factor	SMACNA Architectural Sheet Metal Manual, 7th ed., Tables 1-1 to 1-3
Gutter capacity (gutter size calculator)	Level-gutter equation W = 0.0106·M^(−4/7)·L^(3/28)·(IA)^(5/14); K-style sized as the inscribed rectangle; half-round from Table 1-4; ≤ 50 ft per downspout	SMACNA Architectural Sheet Metal Manual, Chart 1-1 and Table 1-4
Plumbing-code check (gutter size calculator)	GPM = R × A × 0.0104 against Tables 1106.3 and 1106.6, R = 100-year 1-hour rate	2021 International Plumbing Code §1106
Design rainfall by zip	5-min 10- and 100-year, 1-hour 100-year intensities and 24-hour depths per county	NOAA Atlas 14 (PFDS, partial-duration series) at each county's 2020 Census centre of population; zip → county from the Census 2020 ZCTA relationship file. Oregon and Washington not covered (NOAA Atlas 2)

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

Topsoil and fill dirt

Figure	Value	Source
Topsoil weight	0.80–1.15 tons per loose cubic yard	Caterpillar Performance Handbook: topsoil 950 kg/m³ loose, 1,370 kg/m³ in place
Fill dirt weight	1.20–1.35 tons per loose cubic yard	Caterpillar Performance Handbook loose densities: dry sand 1,420 to wet excavated earth 1,600 kg/m³
Compost weight	0.40–0.60 tons per cubic yard	Oregon State University Extension EM 9217: about 1,000 lb/yd³ at 50% moisture, 800–1,200 typical
Garden mix weight	0.60–0.88 tons per cubic yard	Derived: mean of the topsoil and compost ranges for a 50/50 blend
Settling allowance	25% default	Virginia Cooperative Extension: 8–10 in loose topsoil settles to 6–8 in; Caterpillar earth load factor 0.79–0.80
Code fall from the foundation	6 in in the first 10 ft	2021 IRC R401.3
Foundation exposed above grade	6 in, 4 in with masonry veneer	2021 IRC R404.1.6; R317.1 item 5 and R703.7.2.1 for siding and weep screeds
Lawn topsoil cap	4 in default	NMSU Extension H-509: 4–6 in of topsoil after firming

Gravel

Figure	Value	Source
#57 crushed stone	1.35–1.40 tons per loose cubic yard	Ottr Supply (about 1.35); Knife River product sheet (about 1.4 per loose yd³) — the same 1.4 every stone quantity on the site uses
Washed drainage stone (#4–#57)	1.35–1.40 tons per loose cubic yard	Knife River washed rock sheets (about 1.4); Ottr Supply #57
Pea gravel	1.21–1.40 tons per loose cubic yard	Knife River ⅜ × ¼ in rounded rock (about 1.4); GravelShop.com (1.21, a figure that retailer uses for several stones)
River rock	About 1.4 tons per loose cubic yard	Knife River rounded ½–¾ in and 2–4 in rock; no second source found
Crusher run / paver base	1.80–1.86 tons per compacted cubic yard	Wisconsin DOT research report 0092-22-05 (¾-in dense graded base); WisDOT spec 305 pays 1.85
Decomposed granite	No weight published	Supplier pages give only truck and bulk-bag weights; the tool gives volume only
Allowance, clean stone	10%	Same default as the French drain gravel calculator; clean stone has no fines to compact
Allowance, crusher run	40%	Wisconsin DOT: 36–47% more loose than compacted volume for ¾-in dense base; Minnesota DOT pit-gravel rule ×1.3
Allowance, decomposed granite	25%	Kafka Granite pathway spec: allow 20–25% for compaction
Driveway preset	8 in compacted base + 4½ in surface course	Kafka Granite spec (8–12 in base for vehicles); Penn State Center for Dirt and Gravel Road Studies (surface 6–8 in loose, 4½–6 in compacted)
Patio / paver base preset	4 in compacted	Kafka Granite spec (4–6 in for pedestrian traffic); paver makers' own guides govern
Path preset	4 in base + 3 in DG	Kafka Granite (3 in compacted DG over 4–6 in base); Penn State trail bulletin (3 in compacted over 2–6 in base)

Mulch

Figure	Value	Source
Coverage	324 sq ft per cubic yard at 1 in	Clemson HGIC and UF/IFAS mulch fact sheets
Bag sizes	2 and 3 cu ft	Retail packaging
Fine wood and bark mulch	2–3 in	Clemson HGIC; UF/IFAS (no more than 3 in)
Pine bark nuggets	Up to 4 in	Clemson HGIC
Pine straw	2–3 in	Clemson HGIC; UF/IFAS FR052 on how fast it breaks down
Shredded leaves, clippings	No more than 2 in	Clemson HGIC
Rain garden mulch	About 3 in double-shredded	Wisconsin DNR rain garden manual; University of Maine (3–4 in, bark chips float); Vermont DEC (2–3 in)
Setback from walls	6–12 in; under 2 in deep within 12 in of the foundation	Clemson HGIC; UF/IFAS EDIS IN651

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 nearly 290 checks across seventeen 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 2026-09-27