How to Work Out Exact Bag Numbers Without Running Short
Running short of self-levelling compound mid pour means your wet edge dries, leaving you with a cold joint across the room. To fix it you’ve got to wait for the floor to cure, grind the ridge, re-prime and pour a second layer. Calculating self-levelling compound correctly ensures you have exactly the right amount before you start.
Basing an order on simple room dimensions isn’t enough. Subfloor falls, texturing and heating pipe displacement will alter the amount you need. This guide gives you our direct trade formulas – how to map true average depth across a laser grid, apply accurate wastage factors, and order with complete certainty.
The Baseline Calculation Formula
Every manufacturer prints a consumption rate on the technical data sheet. This is expressed in kilograms of dry powder per square metre per millimetre of depth (kg/m²/mm).
For most standard polymer-modified cementitious compounds, the consumption rate sits between 1.6kg and 1.8kg/m²/mm.
Formula:
Area (m²) × Average Depth (mm) × Consumption Rate (kg/m²/mm) = Total Powder Required (kg)
Total Powder (kg) ÷ Bag Weight (e.g., 20kg or 25kg) = Bags Required
Worked Example:
- Area: 40m²
- Target depth: 5mm
- Product rate: 1.7kg/m²/mm (in 20kg bags)
40 × 5 × 1.7 = 340kg
340 ÷ 20 = 17 bags
That gives the baseline figure for a flat slab.
Why You Need More Than a Baseline Figure
If you stop at the baseline calculation though you’re still at risk of running out of material. Real slabs have falls, thirsty textures, and embedded services that swallow compound. To avoid constant second-guessing, build your final order using this method:
- Replace your target nominal thickness with a calculated average depth taken from a laser grid.
- Compensate for the substrate’s condition; mechanical preparation, open pores, and grout joints.
- Adjust the volume up for cable encapsulation or down for wet pipe displacement.
- Add specific safety margins for your batching setup (mixing station vs continuous pump).
Take these four layers in order (as laid out below). Once they’re applied to your initial baseline, you’ll have an accurate bag count that will see you through to the end of the project.
1. Calculating True Average Depth
Don’t use the nominal depth stated on the architectural spec as your calculation figure. Architectural drawings show you design intent – the minimum depth required over the highest point of the slab (such as a 3mm smoothing coat).
In reality, self-levelling compound obeys gravity. It moves to the lowest point of the room first before building any height on the high spots. If your datum requires 3mm of cover over the highest peak, and the slab dips 8mm in the far corner, you are actually pouring 11mm at that low point. Across the full run of the room, your true working average is 5.5mm or higher.
Calculate purely for 3mm, and you will run dry before you reach the doorway.
Find the true average depth before ordering:
- Find the High Point – Set up a rotating laser level and run a receiver across the room to locate the highest point of the substrate.
- Set the Datum – Mark the high point as your minimum cover (e.g., 3mm clearance).
- Map a Grid – Take depth readings on a 1m × 1m or 2m × 2m grid pattern across the bay.
- Average the Depths – Add all measured depths together and divide by the total number of survey points.
True Average Depth (mm) = (Sum of all grid depth points in mm) ÷ (Number of points measured)
Use this calculated average depth in the core formula.
2. Allowing for Substrate Wastage
The condition of your substrate will determine how much of your compound disappears into surface texture before you can achieve any build height. Add these percentage allowances to your dry bag total:
- Power-Floated Concrete (Light Scrabble / CSP 1–2) – Add 5% for general mixing waste and boundary edges.
- Shot-Blasted or Scabbled Concrete (CSP 3–5) – Add 10% to 15%. Open concrete profiles swallow compound simply to fill surface pits.
- Refurbished Tiled Substrates – Add 10%. Grout lines and broken tile lips act as reservoirs that pull down volume across the bay.
- Timber / Plywood Substrates – Add 10%. Factor in slight board deflection, perimeter expansion foam compression, and embedding reinforcement mesh.
3. Factoring in Underfloor Heating (UFH)
Underfloor heating significantly alters compound calculations depending on whether you’re running electric cables or wet pipes.
1. Electric Wire Mats
- You must cover the crown of the heating cable by a minimum of 3mm to 5mm (check the flooring manufacturer’s technical sheet).
- If the heating cable is 3.5mm thick, your target pour depth is 7mm minimum.
- Don’t deduct the wire volume from your calculation; the wire volume is negligible.
2. Retrofit Low-Profile Wet UFH (Over-Floor Panels / Clip Rails)
- Wet pipe setups take up physical volume in the room, displacing compound.
- Calculate the total gross volume required (pipe diameter + minimum cover above pipe crown).
- Deduct approximately 10% to 15% from your total bag calculation to account for the physical water pipes displacing the liquid compound.
4. Application Method - Batch Mixing vs Continuous Pump
- Hand-Mix Stations (2–3 Man Bucket Setups) – Allow 1 extra bag per 50m² for residue left in buckets, mixing paddle fling, and tool scraper losses.
- Continuous Screed Pumps – If you’re hiring a pump, allow 2 to 3 extra bags solely for line lubrication, pump hopper residue, and line washout at the end of the run.
(Bonus) Deep Fills - Bulking Out With Coarse Aggregate
When the survey shows low spots deeper than 10mm to 15mm, pouring neat smoothing compound becomes expensive.
Where permitted by the product’s technical datasheet:
- Bulk out the mixed compound by adding up to 30% to 50% by weight of clean, dry, graded silica quartz or pea gravel (typically 0.8mm–1.2mm or 1mm–3mm).
- For example, adding 6kg of graded aggregate to a 20kg bag increases wet volume yield while holding on to the high compressive strength and reducing overall cost per square metre.
- For localised hollows over 20mm, pre-fill and patch with a rapid-set repair mortar 24 hours prior to the main levelling pour.
Quick-Reference Bag Calculator Matrix
The table below shows bag requirements for standard 20kg bags using a standard consumption rate of 1.7kg/m²/mm (rounded up to the nearest full bag, excluding contingency):
Room Area | 3mm Depth (Smoothing) | 5mm Depth (Standard Level) | 10mm Depth (Deep Fill) | 15mm Depth (Encapsulation) |
10m² | 3 bags | 5 bags | 9 bags | 13 bags |
25m² | 7 bags | 11 bags | 22 bags | 32 bags |
50m² | 13 bags | 22 bags | 43 bags | 64 bags |
80m² | 21 bags | 34 bags | 68 bags | 102 bags |
100m² | 26 bags | 43 bags | 85 bags | 128 bags |
Always round up your order to the next full bag. Having two spare bags on the van isn’t a big problem whereas stopping a pour three buckets short of the threshold will cost you an entire day.
Let Collie Floor Levelling Take The Guesswork Out of Your Subfloors.
From technical data queries and bag estimation to bulk compound supply and fully managed pump pours, the Collie floor levelling team brings two decades of trade experience to your project.
Get in touch with Collie Floor Levelling today for technical advice or an immediate quote on materials and contracting services – 01206 648 158
