Cushion Foam Thickness Calculator

Calculator Inputs

Cushion length

Measure the foam area that carries the load.

Cushion width

Use usable seated width, not piping.

Dimension unit

Both length and width use this unit.

Load per seat

Use body weight plus gear for a worst-case estimate.

Load unit

Per cushion position, not total sofa load.

Cushion type

Back cushions generally need less thickness.

Foam density

Outdoor seating often benefits from 1.8 lb/ft3 or higher.

Density unit

Density affects longevity more than firmness.

Firmness preset

Preset selects a typical ILD value.

Base support

More gaps usually require more thickness.

Daily use level

Higher use increases recommended thickness.

Safety/comfort factor

Typical range: 1.00 to 1.15 for outdoor seating.

Custom ILD (optional)

Use if your foam spec sheet lists ILD.

Reset

Example Data Table

Scenario	Load	Size	Firmness	Density	Base	Recommended thickness
Patio chair, daily	180 lb	20x20 in	Medium	1.8 lb/ft3	Solid	about 4.0 in
Bench, heavy use	220 lb	18x36 in	Firm	2.0 lb/ft3	Slatted	about 4.5-5.0 in
Back cushion	60 lb	22x18 in	Soft	1.5 lb/ft3	Webbing	about 2.0-3.0 in
Daybed, all day	200 lb	28x72 in	Medium	2.2 lb/ft3	Mesh	about 4.0-5.0 in

Examples are illustrative. Use your actual measurements and foam specs for accuracy.

Formula Used

The calculator converts your cushion size into area and estimates average pressure: pressure (psi) = load (lb) / area (in2).

Foam firmness is represented by ILD. ILD is measured over a 50 in2 plate at 25% compression, so psi25 = ILD / 50.

Thickness is estimated from a practical scaling: t approx 4 * pressure / psi25, which simplifies to t approx 200 * pressure / ILD. Multipliers then adjust for cushion type, base support, daily use, density, and your safety factor.

How to Use This Calculator

Measure the load-bearing cushion length and width.
Enter the expected load per seating position.
Select cushion type, base support, and daily use level.
Choose a firmness preset, or enter your foam ILD.
Enter foam density from the product spec sheet if possible.
Click Calculate to see the recommended thickness and notes.
Download CSV to save, or PDF to share.

Load and contact area

Accurate thickness begins with load and contact area. More weight on a smaller cushion footprint raises average pressure, so foam must be thick enough to spread force before you feel the frame. Measure the usable seating surface, not the stitched edge. Enter a realistic per-seat load that includes leaning, shifting, and occasional impact. The calculator converts these inputs into an average pressure used for sizing.

Firmness and ILD selection

Firmness is represented by ILD, the force needed to compress foam to 25 percent on a standard test plate. Lower ILD feels softer but can bottom out sooner under the same load. Higher ILD resists compression and often suits loungers and benches. If you do not know ILD, the preset options provide practical starting points. If you do know it, a custom ILD matches your supplier sheet.

Density and outdoor durability

Density affects how well foam holds its structure over time. Outdoor cushions face heat, moisture, and repeated cycling, which can accelerate softening. Higher density improves longevity and sag resistance. Density is not the same as firmness; it mainly reflects material mass per volume. The calculator applies a density adjustment so low density choices receive extra thickness for durability, while higher density foams may need less thickness for similar support.

Base support and comfort tuning

Support beneath the cushion changes perceived feel. Solid decks spread load evenly, while webbing, slats, and open mesh allow localized bending that increases compression. The model adds thickness for more flexible bases and for heavier daily use. The safety and comfort factor provides extra margin when several users share the furniture, when you prefer a plusher seat, or when you expect long sitting sessions on garden seating.

Using results to choose materials

Use the recommended thickness as a decision tool. For a slimmer profile, choose a firmer ILD or higher density foam, then confirm the compression estimate remains reasonable. For a softer feel, increase thickness or choose a softer ILD while watching bottoming risk. Combine the result with cover fit, batting layers, and drainage design to build cushions that stay comfortable across seasons.

FAQs

What thickness is common for patio seat cushions?

Many outdoor seat cushions land between 3 and 5 inches, depending on load, size, and firmness. Use the calculator when you change dimensions, base type, or foam specs to avoid a cushion that feels too thin or overly bulky.

Does higher density always feel firmer?

No. Density mainly indicates durability and resistance to long term softening. Firmness is better described by ILD. A foam can be high density and still feel soft if its ILD is low.

When should I enter a custom ILD?

Use custom ILD when your foam supplier lists an ILD value on the product sheet. Entering it improves the fit between the recommendation and the material you will actually purchase.

How do slats or mesh affect thickness needs?

Flexible or open supports allow the foam to deflect into gaps, increasing perceived compression. That often requires more thickness than a solid base, especially for benches, loungers, and heavy daily use.

Why does the calculator include a safety factor?

The factor adds margin for comfort and variability, such as different users, uneven seating, or long sitting sessions. Increase it slightly for plushness, or decrease it if you want a firmer, thinner cushion.

Can I use this for back cushions?

Yes. Select the back cushion option. Back cushions carry less vertical load, so the recommended thickness is usually lower. For very upright seating, a moderate ILD often prevents a flat, saggy feel.