What thickness of underlayment should I use?

For self-leveling underlayment, manufacturers usually specify a minimum and maximum thickness, often from 1/8 in (3 mm) up to 1 in (25 mm) in a single pour. For foam or cork underlayments, the thickness is fixed by the product (e.g., 2–6 mm). Always follow the flooring and underlayment manufacturer’s recommendations.

How much waste should I allow for underlayment?

For self-leveling and patching compounds, 5–10% waste is common to account for mixing losses and minor overfill. For sheet, roll, or foam underlayments, 5–10% covers offcuts and overlaps. Complex room shapes or lots of obstacles may require a higher waste factor.

Can I mix units (feet and meters) in this underlayment calculator?

Yes. You can choose feet/inches or meters/centimeters for room dimensions, and you can enter product coverage in either square feet or square meters. The calculator converts everything internally and shows results in both systems.

Underlayment Calculator

Q: How do I calculate how much underlayment I need?

Measure the length and width of the area, multiply to get total square footage (or square meters), then divide by the product coverage per bag or roll. Add a waste factor (typically 5–10%) and round up to the next whole bag or roll.

Estimate bags of self-leveling underlayment, patch, or rolls/sheets of foam and cork underlay for any room, with waste and cost included.

1. Choose underlayment type

2. Room dimensions

Input units:

Length

Width

For multiple rooms, enter the total combined area instead of a single room size.

3. Thickness / product coverage

Thickness units:

Min thickness

Max thickness

Average thickness

Tip: If you know the highest and lowest spots on the floor, set min and max and use the average as a realistic pour thickness.

Product coverage

Coverage at reference thickness (see below).

Reference thickness

Thickness used in manufacturer coverage (same units as thickness above).

Coverage units

Waste / overage

Covers offcuts, mixing loss, and irregular shapes.

Cost per bag / roll

Leave blank if you only need quantities.

Currency

Use $, €, £, etc.

Results

Enter your room size and product details, then click “Calculate underlayment” to see quantities here.

How this underlayment calculator works

This tool is designed for both liquid underlayments (self-leveling compounds and patch/skim coats) and sheet/foam/cork underlayments used under tile, vinyl, laminate, engineered wood, and other floor coverings.

1. Calculating floor area

First we convert your room dimensions into a total area. In imperial units:

Length (ft) = length_ft + length_in ÷ 12

Width (ft) = width_ft + width_in ÷ 12

Area (ft²) = Length (ft) × Width (ft)

In metric units:

Length (m) = length_m + length_cm ÷ 100

Width (m) = width_m + width_cm ÷ 100

Area (m²) = Length (m) × Width (m)

2. Waste / overage

To account for mixing losses, offcuts, and irregular shapes, we apply your waste factor:

Effective area = Area × (1 + Waste% ÷ 100)

For most projects, 5–10% is typical. Use the higher end for complex room layouts or when you are new to self-leveling products.

3. Self-leveling and patch underlayment

Manufacturers usually specify coverage at a reference thickness, for example: “50 ft² at 1/8 in (3 mm) per 50 lb bag”.

Coverage is inversely proportional to thickness. If you pour twice as thick, you cover half the area:

Coverage at your thickness = Coverage_ref × (Thickness_ref ÷ Thickness_avg)

Bags required = Effective area ÷ Coverage at your thickness

The calculator lets you enter minimum, maximum, and average thickness. Only the average is used in the volume and bag calculation, but min and max are shown back to you so you can confirm they are within the product’s allowed range.

4. Sheet, foam, and cork underlayment

For rolls or sheets, we compute the coverage per roll from its length and width:

Roll area = Roll length × Roll width

Rolls required = Effective area ÷ Roll area

You can work in either feet or meters. The calculator converts everything internally and shows the final area in both ft² and m².

5. Cost estimation

If you enter a cost per bag or roll, the calculator multiplies it by the required quantity (rounded up to the next whole unit):

Total cost = Units_required_rounded × Cost_per_unit

Practical tips for choosing underlayment thickness

Check manufacturer limits. Every self-leveling or patch product has a minimum and maximum thickness per lift.
Measure high and low spots. Use a long straightedge or laser to find the deepest low point and the highest point.
Use average thickness for estimating. If your floor ranges from 1/8 in to 1/2 in, an average of around 1/4–3/8 in is often realistic.
Consider multiple lifts. Very deep fills may require a base fill with a different product (e.g., pea gravel + SLC) before a final smoothing layer.

Frequently asked questions

How do I calculate how much underlayment I need?

Measure the length and width of the area, multiply to get the total area, add 5–10% waste, then divide by the coverage per bag or roll from the product data sheet. This calculator automates those steps and handles unit conversions for you.

What if my room is not a perfect rectangle?

You can either break the space into smaller rectangles and add their areas, or measure the total area from a plan. Enter the combined area as if it were one room. The waste factor will help cover irregularities and offcuts.

Can I use this for both tile and vinyl underlayments?

Yes. The math is the same whether you are using cement-based self-leveling underlayment under tile, foam underlayment under laminate, or cork under vinyl. Just be sure to use the correct coverage and thickness limits from the specific product you plan to install.

Is the result a guarantee of performance?

No. The calculator provides planning estimates only. Always read and follow the manufacturer’s technical data sheet, and verify that your substrate, thickness, and environmental conditions meet all requirements.