Methodology — where our numbers come from
Every number on this site traces to a published source, listed below by calculator. Where sources disagree, we show both figures. Where we could not verify a number, we leave it out.
Mulch calculator
Cubic yards = (square feet × depth in inches) ÷ 324, a constant Clemson Cooperative Extension states directly: one cubic yard covers 324 square feet at 1 inch deep.
Depth varies by mulch type and extension office. Shredded bark runs 3 to 4 inches on well-drained sites per Iowa State University Extension, dropping to 2 to 3 inches on heavy soils. Gravel used as mulch gets 1 inch from Clemson and University of Maryland Extension, shallower than some landscaping blogs suggest; we use the extension figure. Bag sizes come from retailer pages: 2 cubic feet (Vigoro) and 1.5 cubic feet (Earthgro) are the two dominant sizes; bags per yard is 27 divided by bag size.
Topsoil calculator
Same formula as mulch. New lawn establishment calls for 4 to 6 inches per Penn State Extension. Raised beds get a wider range: 4 to 8 inches for a finished bed per Clemson, up to 12 to 24 inches for deep-rooted crops like tomatoes per University of Maryland Extension. We show tiers, not one number, since extension offices don't agree.
Gap: no extension source gave a weight-per-cubic-yard figure for topsoil. Two suppliers estimate it by moisture level instead: Topsoil.com puts it at roughly 1,500 to 3,000 lbs per cubic yard, dry to saturated. Ottr Landscape Supply puts it at roughly 2,000 to 2,700 lbs. Both figures are labeled supplier-sourced, not extension-sourced.
Board foot calculator
Board feet = thickness (in) × width (in) × length (ft) ÷ 12, per Woodworkers Source. The catch: this math uses nominal (rough-sawn) thickness, not actual planed thickness. The nominal-to-actual size table (a 2x4 measures 1.5 by 3.5 inches actual) is cross-verified against ProWood and Archtoolbox, which agree on every dimension. Two decking sizes, 5/4x6 and 6/4x6, come from ProWood alone and are flagged single-source.
Deck board calculator
Board count starts from actual width, not nominal. Nominal 6-inch decking runs 5.25 to 5.5 inches actual; we use 5.5, the figure both Decks.com and ProWood support.
Gap spacing is where manufacturers disagree, so the calculator asks which brand you're using instead of hard-coding one number. Trex recommends 3/16 inch side-to-side, widening to 3/8 inch in wooded areas. TimberTech splits by line: AZEK calls for 1/8 to 1/4 inch with no end-to-end gap, while PRO and EDGE call for 1/8 to 7/32 inch. The ranges overlap but aren't the same, so we present both rather than averaging them.
Waste factor runs 10 to 15% extra for straight runs, up to 30% for diagonal patterns, per TimberTech. Joist spacing (16 inches on center standard, 12 for angled decking) traces to Decks.com's framing guide and 2021 IRC Section R507.6. Framing is a code matter: confirm with your permit office before you build.
Fence calculator
Post spacing: 6 to 8 feet is the standard range, with 8 feet the common default because it matches pre-built fence panels, per Barrier Boss. Use 6-foot spacing for heavy panels, high-wind sites, or sandy soil.
Post hole depth is where sources conflict, so we show the range. Quikrete: 1/3 to 1/2 of above-ground post height, plus 6 inches. Sakrete: roughly 1/3 of above-ground height, though its own worked example (an 8-foot fence needs 36 inches) doesn't match that rule, which would call for 32; we report that as published. Penn State Extension gives a flat 36-inch line-post depth and 48 inches for corner and gate posts, with no fractional rule at all. All sources agree the hole should clear the frost line by 6 inches, whichever depth is greater. Confirm with your permit office before setting posts, and call 811 before you dig.
A 50 lb bag of Quikrete Fast-Setting Concrete Mix yields about 0.375 cubic feet; Quikrete sets a 4-inch post in a 10-inch, 2-foot-deep hole with 2 bags, matching Sakrete's 2-bag estimate for a 6-foot post.
Gap we're honest about: the tool calculates posts, pickets, and concrete, but not rails. No manufacturer or extension source publishes a rails-per-section table, so we left rails out rather than invent a rule.
Deck stain calculator
Coverage rates come from manufacturer spec sheets and vary by product, wood condition, and coat number, so the calculator asks for all three instead of using one flat rate.
Cabot's oil-series solid decking stain runs 400 to 500 sq ft/gal smooth, dropping to 250 to 350 sq ft/gal rough. Behr's premium solid color stain covers 200 to 400 sq ft/gal on coat one and 400 to 800 on coat two (direct fetch returned a 403, so this is search-indexed, not a live read). Olympic Maximum Solid states 250 to 350 sq ft/gal, while independent field reviews report closer to 150 to 250 on an aged deck; we use the manufacturer figure and note the gap.
All three agree on the same pattern even though numbers differ: rough wood cuts coverage roughly in half versus smooth, and second coats cover 1.5 to 2 times more area than first coats.
Hardscape tools: gravel, pavers, and retaining walls (coming soon)
We've collected sourced data for gravel, paver, and retaining wall calculators, but the tools aren't live yet. Two gaps show why we're holding these back instead of shipping placeholder numbers.
Gravel density placeholder: one supplier, Gravelshop.com, lists an identical 2,410 lb per cubic yard for every product we sampled, including pea gravel, crushed stone, and river rock. Materials that different shouldn't weigh the same, so we read that as a flat, generic constant and are excluding it. Density will instead come from Stone Center's per-material ranges, such as washed gravel at 1.4 to 1.6 tons/yd³. Stone Center's own crushed-stone figure (2.7 to 3.4 tons/yd³) is roughly double every other row on its own page, so we're excluding that one too; for crushed stone we instead use a broader aggregated industry range of roughly 1.35 to 1.7 tons/yd³.
No extension source for gravel depth: unlike mulch and topsoil, we found no cooperative extension source with a numeric depth recommendation for driveways, paths, or drainage beds. The figures we do have, Braen Supply's 4-inch light-duty base and 2-inch pea gravel minimum, come from a supplier and will be labeled as such when the calculator ships.
Retaining wall rules will carry the same permit language as the fence and deck tools. Walls over roughly 4 feet, per Unilock, typically need geogrid reinforcement, and taller walls should be engineer-reviewed if there's a slope above them. Confirm with your permit office before you build.
Corrections
Found a number that doesn't match its source, or a source that's gone stale? Email us at [email protected]. When we can't verify a number, we leave it out.
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