About Build Calculators

What this site is

Build Calculators is a small set of free calculators for DIY and small contractor jobs: how much concrete, gravel or mulch to buy, and how big an area is. It is an independent project published on runkodo.com. There are no accounts and no paywalls; the site is paid for by the ads on its pages.

The calculators currently on the site:

• Concrete Calculator — Cubic yards and 40, 60 or 80 lb bags for slabs, footings, columns and steps.
• Concrete Slab Calculator — Slab concrete, bag count, rebar grid and gravel base for patios and driveways.
• Gravel Calculator — Cubic yards and tons of gravel, crushed stone or road base for any area.
• Mulch Calculator — How many bags or yards of mulch for beds, rings around trees and playgrounds.
• Square Footage Calculator — Area of rooms, lots and odd shapes in sq ft, sq yd, m² and acres, with cost.

Where the numbers come from

Bag yields

Concrete bag counts use the yields printed on standard US bags of premixed concrete: 0.30 cu ft per 40 lb bag, 0.375 cu ft per 50 lb bag, 0.45 cu ft per 60 lb bag, 0.60 cu ft per 80 lb bag. That works out to 90, 72, 60 and 45 bags per cubic yard. Some high-strength and fast-setting mixes yield slightly less, so check the bag you are buying.

Material densities

Tons are calculated from typical loose, as-delivered densities — the middle of the range most US stone and landscape yards quote. Real material varies with moisture and grading, often by ±10%. The values used everywhere on the site:

• Gravel (general, 3/4"): 2,800 lb per cubic yard (1.4 tons)
• Pea gravel: 2,700 lb per cubic yard (1.35 tons)
• Crushed stone (#57): 2,700 lb per cubic yard (1.35 tons)
• Crusher run / road base: 3,000 lb per cubic yard (1.5 tons)
• River rock: 2,700 lb per cubic yard (1.35 tons)
• Decomposed granite: 2,700 lb per cubic yard (1.35 tons)
• Sand (dry): 2,700 lb per cubic yard (1.35 tons)
• Topsoil: 2,200 lb per cubic yard (1.1 tons)
• Fill dirt: 2,000 lb per cubic yard (1 tons)
• Compost: 1,000 lb per cubic yard (0.5 tons)
• Mulch (shredded hardwood): 600 lb per cubic yard (0.3 tons)
• Wood chips / playground chips: 500 lb per cubic yard (0.25 tons)
• Concrete (wet, normal weight): 4,050 lb per cubic yard (2.03 tons)
• Asphalt (hot mix, compacted): 3,915 lb per cubic yard (1.96 tons)

Rules of thumb

Recommended depths and thicknesses follow common US residential practice and the International Residential Code where it applies (for example, stair risers of at most 7¾ inches). Your local code and your supplier always take precedence.

How the calculations are checked

Every calculator runs on one shared calculation engine. Before each release it is run against a set of hand-checked cases — unit conversions, each shape's volume, bag rounding on exact multiples, the rebar grid and area subtraction — and the reference tables on each page are generated by the same engine, so a table cannot disagree with the calculator above it. All calculations run in your browser; nothing you type is sent anywhere.

Contact and corrections

Found a number that looks wrong, or want a calculator that is not here yet? Email support@runkodo.com. Corrections are fixed and noted with a new “last updated” date. See the privacy policy for how the site handles data and ads.