Tile Calculator

Estimate tiles or boxes needed (with waste) or compute total tiled area. Choose preset tile sizes or enter a custom tile, subtract non-tiled areas, and see the math.

Practical Guide

Concrete Footing Calculator: Fast, Accurate Takeoffs for Spread & Strip Footings

Use this guide to get reliable concrete volumes for isolated (pad) footings, trench/strip footings, and combined footings—then sanity-check results with load-based sizing, waste allowances, and worked examples in US and metric units.

7–9 min read Updated November 10, 2025

Quick Start

  1. 1 In the calculator, select the footing type (isolated/pad, trench/strip, combined, or round) and your units.
  2. 2 Enter the dimensions: length \(L\), width \(B\), and thickness \(t\) (or trench length and cross-section). Add the count if multiple identical footings are used.
  3. 3 Set a waste allowance (typically 5–10% for simple pads; 10–15% for complex trenches, tight forms, or difficult access).
  4. 4 Review the volume output and convert to cubic yards/m³ as needed. Use the “bags of concrete” helper if batching with premix.
  5. 5 Sanity-check with the load method if available: verify that the planned area meets your allowable soil bearing capacity.

Tip: Keep dim inside the formwork. If you measured excavation size, subtract typical form thickness or over-dig before calculating concrete.

Watch-out: This tool estimates volume; it does not confirm code compliance, frost depth, reinforcement, or geotechnical capacity. Engage a licensed professional for design.

Choosing Your Method

Method A — Volume-Only Takeoff

Fastest path for ordering concrete when footing sizes are already specified on drawings.

  • Very quick: \(V = L \\times B \\times t\) (or trench length × cross-section).
  • Ideal for repetitive pads or uniform trenches.
  • Easy to add waste and convert to yd³/m³.
  • Does not check soil capacity or code minimums.
  • Assumes form/over-excavation inputs are correct.
Pad footing: \( V = n \\cdot L \\cdot B \\cdot t \)

Method B — Load & Bearing Check

Validates area against allowable soil bearing \(q_{allow}\) before volume is computed.

  • Detects undersized footings early.
  • Useful for preliminary sizing when loads are known.
  • Requires accurate load \(P\) and \(q_{allow}\) from soils report.
  • Still needs reinforcement and detailing by design professional.
Required area: \( A_{req} = \\dfrac{P}{q_{allow}} \\quad \\Rightarrow \\) choose \(L,B\\) then \(V = L \\cdot B \\cdot t\)

What Moves the Number (Key Drivers)

Thickness \(t\)

The dominant driver of volume for a fixed plan area. Doubling \(t\) doubles \(V\).

Plan dimensions \(L,B\)

Small increases in pad size or trench width scale volume linearly.

Waste & spillage

Form leaks, pump line priming, and over-dig. Use 5–15% depending on complexity.

Excavation vs. form size

Measuring the trench rather than the inside of forms can inflate volumes.

Footing type

Round vs. square pads, strip trenches, and combined footings all yield different formulas.

Soil & frost

Depth to frost and allowable bearing affect thickness and plan area chosen by the designer.

Variables & Symbols

  • \(L\) Footing length (pad side or trench run)
  • \(B\) Footing width (pad side or trench width)
  • \(t\) Footing thickness (depth of concrete)
  • \(n\) Count of identical footings
  • \(V\) Concrete volume
  • \(P\) Service load on the footing
  • \(q_{allow}\) Allowable soil bearing capacity
  • \(A\) Plan area of the footing

Worked Examples

Example 1 — US Units: Pad Footings for a Porch

  • Type: Four isolated square pads under porch posts
  • Dimensions (inside forms): \(L=B=30\\;\\text{in}\), \(t=16\\;\\text{in}\)
  • Count: \(n=4\)
  • Waste: 10%
1
Area per pad: \(A = L\\cdot B = 30\\,\\text{in}\\times 30\\,\\text{in} = 900\\,\\text{in}^2\). Volume per pad: \(V_1 = A\\cdot t = 900\\times 16 = 14{,}400\\,\\text{in}^3\).
2
Convert to ft³: \(14{,}400\\,\\text{in}^3 \\div 1728 = 8.33\\,\\text{ft}^3\). Total for four: \(V = n\\cdot V_1 = 4\\times 8.33 = 33.3\\,\\text{ft}^3\).
3
Convert to yd³: \(33.3 \\div 27 = 1.23\\,\\text{yd}^3\). Apply 10% waste ⇒ \(1.23\\times 1.10 = 1.35\\,\\text{yd}^3\) (order 1.4 yd³).
4
Bag count (80-lb): one 80-lb sack ≈ 0.022 yd³ ⇒ \(1.35/0.022 ≈ 61\) bags (round up).
\[ V = n\,(L\cdot B\cdot t) \quad;\quad 1~\text{yd}^3 = 27~\text{ft}^3 \quad;\quad 1~\text{ft}^3 = 1728~\text{in}^3 \]

Example 2 — Metric: Continuous (Strip) Footing Under a Wall

  • Type: Trench/strip footing
  • Trench length: \(L=18\\,\\text{m}\)
  • Width & thickness: \(B=0.45\\,\\text{m},\\; t=0.35\\,\\text{m}\)
  • Waste: 8%
1
Cross-section: \(A = B\\cdot t = 0.45\\times 0.35 = 0.1575\\,\\text{m}^2\).
2
Volume: \(V = A\\cdot L = 0.1575\\times 18 = 2.835\\,\\text{m}^3\).
3
Waste 8% ⇒ \(2.835\\times 1.08 = 3.062\\,\\text{m}^3\) (order 3.1 m³).
4
Premix bags (25 kg typical): if one bag yields ~0.01 m³ ⇒ ~306 bags (prefer ready-mix).
\[ V = L\,(B\cdot t) \quad;\quad \text{Waste factor} = (1+w) \]

Rounding: Always round up deliveries. A short load costs far more than a small overage left in the chute.

Footing Types & Variations

Different footing geometries change both the volume formula and practical risks.

Footing TypeVolume FormulaTypical WasteNotes on Time / Cost / Safety
Isolated (Pad) — Square/Rect.\(V = n(L\\cdot B\\cdot t)\)5–10%Fast forming; minimal trenching. Watch for uplift/frost requirements and pedestal keys.
Round Pad\(V = n\big(\\pi (D/2)^2 t\\big)\)6–12%Sonotube-style forms; easy to place but depth control matters.
Strip/Trench\(V = L(B\\cdot t)\)8–15%Lengthy excavation; stability of trench walls is a safety concern. Shoring may be required.
Combined/StrapSum of individual pad volumes + strap beam10–15%More forming and steel; often used near property lines or eccentric loads.
Mat/Raft (simple est.)\(V = L\\cdot B\\cdot t\)5–8%Engineering-driven; not usually DIY. Pumping and finishing logistics dominate.
  • Confirm that bottom of footing is below frost depth or protected per local code.
  • Use clean, undisturbed soil or properly compacted base; remove loose spoil.
  • Rebar, chairs, and cover must match the structural details.
  • Provide keyways/dowels where walls or columns will be cast later.

Buying, Logistics & Checks

Selection Criteria

  • Mix design: Strength (e.g., 3000–4000 psi / 20–28 MPa) and exposure class per climate.
  • Slump/workability: Enough for placement without segregating; avoid over-watering.
  • Aggregate size: Match bar spacing and cover to prevent honeycombing.
  • Admixtures: Set retarders for hot days; air-entrainment in freeze–thaw regions (per spec).

Logistics & Installation

  • Confirm access for the truck or schedule a line/pump.
  • Place on moist, firm base; avoid standing water. Screed, bull-float, and consolidate around bars.
  • Curing: Cover and keep damp; protect from rapid drying and freezing.
  • Have rebar, chairs, anchors, and dowels tied and inspected before the truck arrives.

Sanity Checks

  • Compare total volume versus similar jobs; big gaps often mean a unit or dimension error.
  • Double-check that calculator inputs are inside-of-form sizes, not excavation width.
  • If using the load method, confirm \(P\) and \(q_{allow}\) source and safety factors.

Codes & standards: Local building codes govern minimum sizes, frost depth, concrete strength, and reinforcement. This article is informational and not a substitute for stamped design or inspection.

Frequently Asked Questions

How much waste should I include for footings?
For simple pads with straightforward access, 5–10% is typical. For long trenches, tight forms, or pumped placements, 10–15% is safer. Poor subgrade or rework pushes waste higher.
Does the calculator account for reinforcement and soil capacity?
No. The tool outputs concrete volume. Reinforcement layout, allowable bearing, frost depth, and other design checks must follow drawings and local codes or a licensed engineer’s direction.
What’s the difference between excavation volume and concrete volume?
Excavations are usually wider than the finished footing (over-dig, sloughing). Enter the inside-of-form dimensions to estimate concrete. If you only have trench dimensions, subtract typical form/over-dig.
How do I size a footing from loads?
Use \(A_{req} = P/q_{allow}\). Choose \(L,B\) to meet or exceed \(A_{req}\), then apply the specified thickness. This is a preliminary check—final sizes depend on settlement, uplift, and detailing.
Can I convert the result to bags of concrete?
Yes—divide the required yd³ or m³ by your bag yield (e.g., an 80-lb bag ≈ 0.022 yd³). For volumes above ~1 yd³ (0.75 m³), ready-mix is typically more economical and consistent.
How do round footings differ?
Use \(V = \\pi (D/2)^2 t\) for each footing, then multiply by count and add waste. Round forms are efficient but ensure proper cover and bar spacing near the curved face.

Scroll to Top