Introduction
A bid is a promise made of quantities. For footing excavation, the quantities are trench volume (bank, for digging), loose volume (for hauling), backfill volume (for the return trip), and the labor or machine hours each one consumes. Get the takeoff right and the bid prices itself; get it wrong and you either lose the job or lose money doing it.
The Footing Excavation Calculator produces the three volumes every excavation bid needs. This guide covers the workflow around it: reading the plans, measuring the site, choosing the method, converting volume to hours, and assembling the bid — the estimator’s path from drawing to number.
The Takeoff Process: Plans to Quantities
- Read the foundation plan. Pull the footing schedule: footing width and depth per run, any interior footings, thickened slab edges, and stepped sections. Note the scale and confirm it against a known dimension.
- Trace the trench length. Measure each footing run’s centerline or outside line consistently; split L- and T-shapes into non-overlapping rectangles so corners count once. Add interior footing runs to the total.
- Set the trench width. Footing width + 15–30 cm (6–12 in) working room per side, per run. Wider in caving soil.
- Set the trench depth. Footing depth + over-excavation, checked against frost-line and bearing requirements from the plans and local code.
- Walk the site. Verify plan dimensions against the ground, probe the soil at each corner, note access constraints, overhead lines, and stockpile or truck-staging space. Adjust overdig and swell to what you find — the material guide maps soil to factors.
- Compute the three volumes. Bank (digging), loose (hauling), backfill = bank − footing concrete. These are the bid’s quantity backbone.
Key Terms and Variables
Trench Length (L)
Total footing run from the plans, verified on site. The takeoff’s most error-prone measurement — every downstream quantity scales with it.
Production Rate
Volume per hour or day for the chosen method. Planning figures: a mini-excavator with operator typically moves roughly 10–20 bank m³ per day in ordinary soil; hand digging runs roughly 1–2 m³ per person-day. Adjust for soil, depth, and access — these are planning figures, not guarantees.
Overdig % and Swell Factor
Set from site conditions, not defaults, once you’ve walked the ground: 10% overdig and ×1.25 swell suit firm ordinary soil; raise both for caving, wet, or rocky ground.
Mobilization
The fixed cost of getting machine and operator to site — often a half-day minimum. Small trenches are dominated by mobilization, which is why they price by the day rather than the yard.
The Formulas Involved
Formula 1: The three volumes
Bank = L × W × D × (1 + o); Loose = Bank × s; Backfill = Bank − V_footing. Run these first — everything in the bid hangs off them. The derivation is in the formula-explained guide.
Formula 2: Unit conversion
1 m³ = 1.308 yd³; 1 yd³ = 27 ft³. Bid in the units your market quotes — usually cubic yards in the US — and keep labor, machine, and disposal all in the same system.
Formula 3: Hours and cost
Machine-days = Bank ÷ daily production; Labor cost = hours × burdened rate; Bid = (quantities × unit rates) + mobilization + disposal + contingency. Carry 10–20% contingency on uncertain ground, more with rock risk. Pricing context for each line is in the cost guide.
Worked Example: Bidding a 44 m Trench
Take the standard trench — 44 m perimeter, 1.0 m wide, 0.55 m deep, 10% overdig, 1.25 swell: 26.62 m³ (34.82 yd³) bank, 43.53 yd³ loose → 4 loads, backfill 14.74 m³ (19.28 yd³). Bid it with a mini-excavator at a planning production rate of 15 bank m³/day:
Step 1 — Machine time. 26.62 ÷ 15 = 1.77 → 2 machine-days (plus mobilization).
Step 2 — Digging line. 34.82 yd³ × illustrative $100/yd³ = $3,482.
Step 3 — Haul-off. 4 loads × illustrative $450 = $1,800.
Step 4 — Backfill and compaction. 19.28 yd³ × illustrative $70/yd³ = $1,350.
Step 5 — Contingency and total. Subtotal $6,632 + mobilization (say $400) = $7,032; + 15% contingency = ~$8,087 bid (illustrative — local rates rule).
Cross-check in imperial — 34.82 yd³ at $100/yd³ = $3,482 regardless of system; 26.62 m³ = 34.82 yd³ confirms the quantity the rate multiplies. The cross-check that matters in bidding is unit consistency: a metric quantity times an imperial rate is the classic bid error.
Practical takeaway: the takeoff produces quantities; production rates convert quantities to time; time and unit rates convert to money. If any bid you review skips a step — quantities straight to a lump sum with no production math — ask to see the missing step. Method selection (hand vs. machine) is compared in the DIY vs pro guide.
Common Variations
- Stepped or sloped sites: take off each step separately — production rates drop on slopes, so price deep sections at lower daily output.
- Rock encountered: stop and re-price; rock is a different operation (ripping/breaking) at a different rate, usually hourly. Never absorb it into the soil rate.
- Tight access: smaller machine, lower production, more days — and spoil goes straight to trucks, so haul-off scheduling becomes critical path.
- Owner-supplied disposal: if the owner takes spoil, the haul line shrinks to trucking only — clarify who pays tipping fees in writing.
- Winter or wet conditions: cut production rates and raise overdig; frozen or saturated ground is a different job than the plans describe.
Mistakes That Distort Results
- Taking off from plans without a site walk. Grade, soil, and access live on the site, not the drawing. Every one of them moves the bid.
- One production rate for all conditions. Rocky, deep, or tight work produces at a fraction of open-ground rates. Segment the takeoff by condition.
- Forgetting mobilization on small jobs. A half-day minimum can exceed the digging cost on a tiny trench — price it explicitly.
- Bidding loose yards at bank rates. Haul-off quantities are loose; digging quantities are bank. Keep the two — and their rates — separate.
- No rock contingency. The estimator’s classic ruin: hitting rock on a fixed-price soil bid. Probe, and carry contingency.
- Lump-sum bids with no quantity backup. If the scope changes (deeper footings, added runs), a quantity-based bid adjusts cleanly; a lump sum becomes an argument.
The full error catalogue is in the common mistakes guide.
Related Guides
- Cost Guide — planning ranges for each bid line.
- Formula Explained — the quantity math, derived step by step.
- Pre-Dig Checklist — the site-walk sequence before you bid.
- Pro Tips — field habits that protect the estimate.
- Complete Guide — the end-to-end volume workflow.
Frequently Asked Questions
How do I estimate excavation from construction plans?
Pull footing width, depth, and runs from the foundation plan; compute trench length (corners once), width (footing + working room), and depth (footing + over-excavation, frost-checked); then walk the site to verify dimensions, soil, and access before finalizing overdig and swell.
How long does it take to dig footing trenches?
As planning figures, a mini-excavator with operator moves roughly 10–20 bank m³ per day in ordinary soil — so a 26.6 m³ trench is about 2 machine-days. Hand digging runs roughly 1–2 m³ per person-day. Soil, depth, and access move these significantly.
Should excavation bids be per yard or lump sum?
Build the bid from quantities × unit rates (per yard for digging, per load for hauling) plus mobilization and contingency — then present whatever total format the client wants. Quantity-based backup lets the price adjust cleanly when scope changes.
How much contingency belongs in an excavation bid?
10–20% for known ordinary soil; more when rock is possible, the soil is untested, or access is difficult. Excavation has the highest subsurface uncertainty of any trade — price the uncertainty, don’t absorb it.
What should I clarify before bidding haul-off?
Who pays tipping fees, the truck capacity assumed, whether spoil can be stockpiled or goes straight to trucks, and the haul distance. Each one changes the haul line.
Get Your Number
Estimating is takeoff plus judgment: quantities from the plans and the site walk, production rates from the method and the soil, and money from local rates with an honest contingency. The Footing Excavation Calculator gives you the bank, loose, and backfill volumes the whole bid rests on. Verify code-driven depths with your local building authority, get the disposal terms in writing, and never bid rock ground at soil prices.
Disclaimer: Calculator results are planning estimates based on the dimensions you enter. Actual excavation volumes vary with site conditions, soil type, and trench accuracy. Verify volumes on site and consult a licensed professional or your local building authority for structural, soil, or code-related decisions.
