How to Estimate a Concrete Slab in 5 Minutes
A step-by-step walkthrough of estimating a slab-on-grade using Seleri: from dimensions to a complete bill of quantities with concrete, rebar, and labor.
A slab-on-grade is one of the most common elements in residential and commercial construction — and one of the most straightforward to estimate once you have the right tool. Here's how to do it in Seleri in under five minutes.
What you need to know before you start
Before opening the estimator, gather three key dimensions:
- Slab length and width — the footprint in meters
- Slab thickness — typically 10 to 15 cm for residential slabs
- Rebar specifications — diameter (usually HA 8 or HA 10) and spacing (typically 15 to 20 cm on center)
You'll also want to know whether a gravel sub-base is required, and if so, how deep.
Step 1 — Select the slab template
In Seleri, open the template picker and choose Slab on Grade. This template covers:
- Concrete volume (accounting for slab thickness)
- Rebar quantity in both directions (as a mesh)
- Gravel sub-base volume
- Polyane moisture barrier
- Perimeter formwork
- Labor for pouring and finishing
Step 2 — Enter your dimensions
Fill in the input fields:
| Input | Example value |
|---|---|
| Length | 10.00 m |
| Width | 8.00 m |
| Thickness | 0.12 m |
| Rebar diameter | HA 10 mm |
| Rebar spacing | 20 cm |
| Gravel bed depth | 10 cm |
The template derives intermediate quantities automatically — slab area (80 m²), concrete volume (9.6 m³), total rebar length — and feeds them into the BOQ.
Step 3 — Review the bill of quantities
Click Generate. Seleri produces a complete BOQ:
- Concrete 350 kg/m³ — volume in m³, ready for mixer ordering
- Rebar HA 10 — total weight in kg, including overlap allowance
- Gravel — volume in m³ for the sub-base
- Polyane film — area in m²
- Formwork — perimeter length in m
- Labor — pouring + finishing, and formwork installation
Each line shows quantity, unit price, and total cost. You can adjust unit prices to match local market rates.
Step 4 — Adjust and iterate
Need to check what happens if you increase the slab from 12 cm to 15 cm? Change the thickness field — the concrete volume, rebar weight, and costs update instantly. No formula hunting, no manual recalculation.
This instant recalculation is the core value of a parametric tool: you're exploring options, not managing a spreadsheet.
Common mistakes to avoid
Forgetting the sub-base. A well-compacted gravel layer protects the slab from differential settlement. Don't skip it in the estimate.
Using 300 kg/m³ concrete instead of 350. For slabs subject to foot traffic and light loads, 350 kg/m³ is standard. Using a lighter mix might save cost initially, but leads to cracking.
Ignoring edge formwork. It's a small line item, but it's real labor and real material. Parametric templates include it automatically.
What comes next
Once you have a slab estimate, you typically need:
- A strip footing underneath load-bearing walls
- Block wall estimates for the perimeter
- Slab finishing (screed, tiles, or polished concrete depending on use)
Seleri has templates for each of these. You can create one project and add multiple estimates — one per construction element — to build up a full project cost.
Open the estimator and try the Slab on Grade template with your own dimensions.