How do I price a concrete slab without underquoting the prep?

Price a concrete slab by costing the prep as separate line items, not a lump sum. Break excavation, compaction, formwork, reinforcement, concrete supply, finishing and disposal into individual lines, add your margin to each, and present the total as a single price. The prep is where most underquoting happens because concreters estimate it as one block instead of measuring each task against what the site actually needs.

Why does the prep eat your margin, not the pour?

The pour is the visible part of a slab job, but its cost is predictable: concrete is priced per cubic metre, the pump hire is a known rate, and finishing time scales with area. The prep is where the variance hides. Excavation depth depends on what the site survey reveals. Compaction effort depends on soil type. Formwork complexity depends on the shape, the fall and whether you are boxing around services. Reinforcement depends on the engineer's spec or the NCC deemed-to-satisfy tables.

When these tasks get lumped into a single "site prep" allowance, the estimate is usually based on the last job, not this one. A sloping block, clay soil, or restricted access can double the excavation hours without changing the slab area at all. That difference comes straight off your margin.

Specialty trade contractors typically operate on gross margins of 15 to 25 per cent¹. On a $15,000 slab, a 25 per cent margin is $3,750. Underquote the prep by two days of bobcat hire and tip fees and most of that margin is gone before you have poured a drop.

How do I break the prep into priceable line items?

Your customer sees one price. Internally, every slab quote should carry at least these seven lines. Each one gets its own cost estimate and margin, so you can see exactly where the money sits and where it leaks.

This is your internal pricing sheet. The customer quote shows a single lump sum or a simple breakdown (prep, concrete, finishing). The line-item detail stays in your system so you can compare actual cost to estimated cost after the job and tighten the next quote.

Weather is one of the biggest variables in that gap. For a step-by-step process, see our guide to handling weather delays on a concrete pour.

What should I measure on site before I quote?

A slab quote built from a phone call or a set of plans alone is an estimate, not a price. The site visit is where you catch the variables that move prep cost. Measure these before you commit to a number:

• Fall across the pad. Use a laser level or a dumpy. A 300mm fall on a 10 m pad means significantly more excavation on one end, or a thicker slab with deeper edge beams.
• Soil type. Sandy loam excavates fast and compacts well. Heavy clay is slower to dig, swells more on the truck, and may need lime stabilisation or extra imported base.
• Access. Can a bobcat or mini excavator reach the pad? If you are barrowing spoil 30 metres because the machine cannot get through, that is a full day of labour you need in the price.
• Services. Sewer, stormwater, gas and electrical routes across the slab footprint mean penetrations, boxing and possibly hand-digging near live services. Before any excavation, lodge a request with Before You Dig Australia (call 1100, free) to get utility plans.
• Tip distance. Spoil disposal is priced per tonne, and the tip fee plus cartage varies by suburb. A 40 km round trip to the nearest clean-fill facility versus a 10 km one changes the disposal line by hundreds of dollars.

If you cannot visit the site, price from plans plus a contingency line that covers the unknowns, and state that the price is an estimate subject to site inspection. As the ACCC states: "A quote is a fixed price. If you give a quote, you can't charge more than the quoted amount." Calling a non-fixed figure a "quote" then adding costs later is a compliance risk under Australian Consumer Law.

How do I handle variations once the dig starts?

Rock, roots, contaminated fill, an unmarked sewer line: the things that blow a slab budget usually appear after the excavator starts. The fix is a written variation process, agreed before work begins and documented in your job-management system.

• State the variation clause in your contract. "Any additional work outside the agreed scope will be quoted separately and requires your written approval before we proceed." This is not just good practice, it protects you under the ACL. A price you call a quote is binding, so new scope needs a new agreement.
• Photograph and document. When the excavator hits rock or an unexpected service, photograph it, note the time, and send the customer a variation request with a cost and a timeline impact before you keep digging.
• Use your system. Job-management tools like ServiceM8, Tradify or Buildxact let you raise a variation inside the job, get a digital sign-off on the customer's phone, and attach it to the invoice. That paper trail protects both sides. For a deeper look at how quoting tools connect to your pricing, see our guide on quoting jobs faster.

Scope-creep leakage, where extra work gets done and never charged, is one of the common sources of margin loss in trade businesses, with variation recovery estimated at 3 to 8 per cent of revenue routinely absorbed (trade software vendor data, directional estimate, low confidence). Written variations recover that revenue and keep the customer relationship clean.

What does a realistic slab pricing example look like?

This is a simplified internal pricing sheet for a 50 m² residential garage slab on a flat suburban block in south-east Queensland, standard 25 MPa concrete, SL82 mesh, broom finish. All figures are directional estimates for illustration, not a quote for any specific job.

Residential concreting jobs commonly range from $3,000 to $25,000 (trade cost guides, directional). This example sits in the lower half because it is a simple flat pad. Add slope, restricted access, exposed-aggregate finish or deeper footings and the prep lines climb fast.

How do I track whether my estimates are right after the pour?

A quote is a prediction. The only way to know whether it was accurate is to compare it to the actual cost after the job closes. This takes five minutes if your system tracks it.

• Record actual hours. Log start and finish times for each prep task (dig, form, steel, pour, finish). Compare to the hours you estimated.
• Record actual material costs. Concrete invoice, mesh, timber, tip receipts, crushed rock delivery. Compare to the line-item estimates.
• Calculate actual margin. Revenue minus actual cost, divided by revenue. If you quoted 20 per cent margin and delivered 8 per cent, the prep estimate was the likely culprit.

Job-costing dashboards in tools like Buildxact, Tradify or ServiceM8 automate this comparison. Over ten jobs, the pattern becomes clear: maybe you consistently underquote excavation on clay, or you never price spoil swell accurately. That pattern is worth more than any pricing formula because it is built from your own real numbers.

If you are spending hours each week on this kind of admin, that is a systemic problem worth solving once. Book a free 15-minute discovery call and we will walk through where your quoting and job-costing gaps sit.

What are the safety and compliance costs to include?

Some prep costs are not optional. They are regulatory requirements, and missing them from a quote means either absorbing the cost or skipping the step, neither of which ends well.

• Before You Dig (BYDA) lodgement. Free to lodge, but if you hit a service you did not locate, you carry the strike liability and a potential WHS breach. Build the time to lodge and wait for responses into your schedule.
• Safe Work Method Statement (SWMS). Excavation deeper than 1.5 metres is classified as high-risk construction activity under the model WHS Regulations, requiring a Safe Work Method Statement (Safe Work Australia). Deep footings on sloping sites can hit this threshold. Factor in the time to prepare or update the SWMS.
• Sediment and erosion control. Council may require silt fencing, bunded stockpiles and stormwater protection. The material cost is small, the non-compliance risk is not.
• Engineering for non-standard slabs. Reactive soil classifications (H, E, P classes under AS 2870) and slabs near boundaries, on fill, or carrying point loads need an engineer's design. That design fee is a project cost, not overhead.

Licensing requirements for concreting also vary by state. In Queensland, a QBCC trade contractor licence is required. In New South Wales, a contractor licence is required for work over $5,000 (labour and materials including GST, NSW Fair Trading threshold). In Victoria, concreting falls under the domestic builder registration regime rather than a standalone trade licence. In Tasmania, concreting is explicitly unlicensed. Claiming "licensed" in a state where no such licence exists is misleading under ACL s18 and s29. For more on how licensing affects trade businesses, see our guide on getting more concreting jobs in Australia.

What mistakes cost concreters the most margin on prep?

Five recurring mistakes, drawn from common trade contractor pain points:

• Quoting excavation from the plan, not the site. Plans show the slab footprint. They do not show the 400mm fall, the rock ledge at 200mm, or the mature tree root system along one edge. The site visit catches what the plan cannot.
• Ignoring swell factor on spoil. Excavated soil expands. One cubic metre in the ground becomes roughly 1.3 cubic metres on the truck. If you price disposal on in-situ volume, you underquote by about 30 per cent on that line.
• Lumping formwork into "prep." A rectangular pad with no step-downs is half a day. Add a step-down for a garage-to-house junction, boxing for three plumbing penetrations and a curved edge, and formwork is a full day with more timber. Price it separately.
• Not pricing the second visit. Strip formwork, backfill edges, and clean up. If you do not price this half-day, you are working for free the morning after the pour.
• Absorbing small variations. The customer asks you to extend the slab 500mm while you are already set up. That extra half-metre needs more excavation, more base, more concrete and more mesh. Around 40 per cent of small business ombudsman assistance requests relate to slow-paying debtors (ASBFEO, directional), and unbilled variations feed that cycle: work done, never invoiced, never paid.