Foam Grades Explained: Density, Firmness and Cushion Foam
The easiest way to choose cushion foam is to start with the application, then thickness, then feel. If you are replacing an insert, measure the cover first using the cushion cover measuring guide, then use the foam cut to size calculator once you know the size and how the foam will be used.
In a foam grade (e.g. N31-190), the first number points to density and the second number points to hardness or firmness. Density is about durability and how much foam material is in the cushion. Hardness is about how firm the cushion feels under load.
Workshop note from Home Upholsterer
Home Upholsterer has supplied upholstery foam since 1994. For DIY customers, the most useful question is not "what is the firmest foam?" or "what is the best foam?" It is "what foam suits where this cushion will be used?"
Quick Selector: Which Foam Grade Should I Start With?
| Use case | Common starting point | Why |
|---|---|---|
| Everyday sofa seat | N31-190 or N30-130, depending on thickness and feel | Good domestic seating options when matched to the cushion depth |
| Sofa back cushion | N23-130 or another softer back-cushion option | Backs usually carry less load than seats |
| Dining chair or thin seat | H35-200 or another firmer seating grade | Thin seats need firmer support to reduce bottoming out |
| Mattress or overlay | Use mattress or overlay recommendations, not seat rules | Lying support is different from sitting support |
| Outdoor cushion exposed to rain or wet swimmers | DF-FIRM or suitable reticulated outdoor foam | Reticulated foam drains and dries better than standard indoor foam |
These are starting points, not universal rules. Use the application dropdown in the foam cut to size calculator to choose foam grade for custom cushions based on the shape, thickness and intended use.
What Does Foam Density Mean?
Foam density is measured in kilograms per cubic metre, written as kg/m3. In simple terms, it tells you how much foam material is in the cushion. Higher density is usually linked with better durability and longer-lasting support.
Density does not tell you everything about comfort. A high-density foam can still be soft, medium or firm depending on its hardness rating. That is why density and hardness need to be read together.
What Does Foam Hardness or Firmness Mean?
Hardness describes how firm the foam feels when it is compressed. In many Australian foam grade codes, the second number is the hardness or firmness figure. A higher number generally means a firmer feel.
Firmness is not the same as quality. A very firm low-density foam may feel strong at first but lose shape sooner than a better-density foam chosen for the right job.
How to Read a Foam Grade Code
A grade such as N31-190 can be read in two parts:
- N31: the density reference, around 31 kg/m3.
- 190: the hardness or firmness reference.
This is why simply asking for "high density foam" is not enough. You still need to know whether the cushion should feel soft, medium or firm, and whether the thickness suits the job.
Application Comes Before Personal Feel
Personal comfort matters, but the cushion first has to suit the job. A sofa seat needs more support than a sofa back. A dining chair pad is usually thinner than a lounge cushion and may need a firmer grade. Outdoor furniture that gets wet should use outdoor reticulated foam rather than standard indoor cushion foam.
Once the application is right, then you can decide whether you prefer a softer, medium or firmer feel.
Thickness Changes How a Grade Feels
The same grade can feel different at different thicknesses. A soft grade in a deep cushion may feel comfortable because there is enough foam to compress. The same soft grade in a thin seat can bottom out, making the cushion feel unsupportive.
For replacement cushions, measure the cover thickness and choose the grade with that thickness in mind. If the seat is thin, lean toward firmer support. If the cushion is deep and you want a softer sit, a medium option may work better. The foam thickness guide explains this support tradeoff in more detail.
Indoor Foam vs Outdoor Reticulated Foam
Standard indoor upholstery foam is suitable for many sofas, chairs, daybeds and cushions that stay dry. If the cushion will be exposed to rain, splashes, pool use or marine conditions, use outdoor reticulated foam instead.
Reticulated foam has a more open structure so water can move through it. That makes it a better choice for outdoor cushions that actually get wet, not just cushions used under cover.
How to Choose Foam Online
- Measure the cushion cover, not the old compressed foam.
- Choose the closest shape in the custom foam calculator.
- Enter the thickness in millimetres.
- Select the application, such as sofa seat, sofa back, dining chair seat or outdoor furniture.
- Compare the recommended grades and choose the feel you want.
- Add Dacron wrap if you want a rounder, fuller finish.
If you want to browse the wider range before ordering, see foam and fillings or read the foam FAQs.
Foam Grades FAQs
What is the difference between foam density and firmness?
Density is about how much foam material is in the cushion and is closely linked with durability. Firmness, also called hardness, is about how soft or firm the foam feels when compressed.
Is high density foam always firm?
No. High density and firm are not the same thing. A foam can be higher density but still have a softer or medium feel depending on the hardness rating.
What foam is best for sofa seat cushions?
N31-190, N30-130 and H35-200 are common starting points for domestic sofa seats, depending on cushion thickness, use and preferred feel. Use the calculator application dropdown to narrow the options.
What foam is best for sofa back cushions?
Back cushions often suit softer options than seat cushions because they carry less body weight. N23-130 can be a sensible starting point for many back-cushion uses.
What foam should I use outdoors?
If the cushion will get wet, choose outdoor reticulated foam such as DF-FIRM or another suitable dry-flow style foam. Standard indoor foam is not the best choice for cushions exposed to rain or pool use.
Can I get high density foam cut to size?
Yes. Use the foam cut to size calculator to enter your cushion dimensions, choose the application and select a suitable foam grade for your project.