How we calculate

Principles

• Show the math. Every calculator has a step-by-step breakdown with your own numbers and a list of assumptions.
• No invented data. We don't publish material prices we can't source. Costs use the prices you enter.
• Round only at the end. Volumes are kept at full precision. Counts of things you buy (bags, posts, boards) are rounded up to whole items.
• Say what's not included. Each calculator lists its limitations: what it does not cover and when to ask a professional.

Units

All conversions use the exact international definitions: 1 inch = 2.54 cm and 1 foot = 0.3048 m. One cubic yard is 27 cubic feet, and one cubic foot is 0.028316846592 m³ (0.3048³).

Source: National Institute of Standards and Technology,U.S. Survey Foot (the international foot of 0.3048 m exactly is used in all applications) andSpecial Publication 811.

Measurements can be typed as you read them from a tape: 10' 6", 10 ft 6 in,5 1/2, 5½, 3 m or 30 cm. A number after feet with no unit is read as inches (10' 6 is 10 ft 6 in). Anything ambiguous, like 10 6, is rejected rather than guessed.

Concrete

Slab, footing:   length × width × depth
Round column:    π × (diameter ÷ 2)² × height
Post hole:       π × (hole ÷ 2)² × depth − post cross-section × depth
Total:           volume × quantity × (1 + waste %)
Bags:            total ÷ yield per bag, rounded up

For post holes you choose whether the post's volume is subtracted. When it is, the concrete is assumed to fill the hole around the post for the full depth you enter. A square post only fits a round hole if its diagonal (side × √2) is smaller than the hole, and the calculator checks this.

Bag yields

Source: QUIKRETE®, Concrete Mix (No. 1101) data sheet, revised October 2022. Yields differ between products, so the yield can be edited in the calculator.

The slab calculator also shows the total rounded up to the next quarter cubic yard as a convenience. Suppliers' order increments vary; ask yours.

Fences

Boards & rails:
  sections = length ÷ max post spacing, rounded up (posts spaced evenly)
  posts    = sections + 1
  boards   = (length + gap) ÷ (board width + gap), rounded up
  rails    = sections × rails per section
Panels between posts:
  sections = (length − post width) ÷ (panel + post width), rounded up
  last panel = whatever is left, cut to fit
Panels on post faces:
  sections = (length − post width) ÷ panel width, rounded up
  last panel = whatever is left, cut to fit
Post concrete:
  π × (hole ÷ 2)² × depth − post width² × depth, per post

For panels between posts, if a run leaves less than one post width, there is no room for another post, so the calculator uses full panels and tells you how much extra space to spread across the bays instead of suggesting a sliver of panel. Panel products differ, so measure the actual panel and follow its installation guide.

"Auto" rails per section follows manufacturer installation guides: 2 rails under 6 ft, 3 rails from 6 ft and 4 rails from 8 ft (UFP fence installation instructions,wood fencing guide). It is not a code requirement, and you can override it. Post spacing guidance of no more than 8 ft comes from fence installation guides such as thisredwood privacy fence guide.

Lumber sizes are entered as actual dimensions. Dry dressed sizes such as 1×6 = ¾ × 5½ in, 4×4 = 3½ × 3½ in and 6×6 = 5½ × 5½ in come from the American Softwood Lumber Standard,NIST PS 20-25, Table 3. Retail fence pickets vary (often ⅝ in thick and 5½–5⅝ in wide), so measure the boards you buy.

Post depth and post concrete

By default the fence calculator sets posts at one-third of their total length in the ground, which is half their height above ground. This follows QUIKRETE's post-setting instructions, which also call for a hole about three times the post's width and about 6 in of gravel below the post. The gravel is not included in the concrete total. Frost depth, soil and local codes can require deeper posts; you can enter your own depth.

Post concrete can be counted in 40, 50, 60 or 80 lb bags. QUIKRETE Fast-Setting Concrete Mix yields the same per size as standard mix: 60 lb = 0.45 cu ft (data sheet) and 50 lb ≈ 0.375 cu ft (spec sheet).

Digging safety: in the US, contact 811 before digging so underground utilities can be marked.811 Before You Dig.

Waste allowances

Waste is a percentage you control. It is added to concrete volume, boards, rails and post concrete. It is not added to posts or panels, which are counted exactly. The defaults (10%) are starting points, not recommendations for your specific job. For ready-mix, the National Ready Mixed Concrete Association suggests ordering 4–10% more than the calculated volume (CIP 8,CIP 31); applying a similar margin to bagged concrete is our assumption.

How the math is tested

Every formula lives in plain TypeScript functions, separate from the page, and is covered by automated tests. The tests include:

• worked examples like the ones on each calculator page
• the same physical size entered in feet, inches, meters and centimeters, which must give identical results
• floating-point edge cases (for example, an exact 16 ft run must not round up to an extra section)
• empty, negative, zero and unreasonable inputs
• tape-measure notation and fractions

The tests run before every build.

What we don't do

• We don't give structural, engineering or code advice. Thickness, depth, reinforcement and permits depend on your project and local rules.
• We don't show "average prices" without a verifiable, dated source.
• We don't keep a database of what you type. Values you change are kept in the page address so results can be reloaded or shared; see the Privacy Policy for what that means.

Corrections

Found a number that looks wrong? Tell us what you entered and what you expected. We check every report against the formulas above.

Calculators: Concrete ·Concrete Slab ·Fence