How to Buy a Memory Foam Mattress Online: A Practical Guide
Mattresses in a box have made it easy to buy a bed online, but the choice can be overwhelming and you cannot lie on it before it arrives. This guide explains how to narrow down thickness and firmness for your body and sleep style, what to check about cooling and certifications, and what kind of frame or foundation you need to go with it.
Affiliate disclosure: HashMonkey is reader-supported. We earn a commission if you buy through our Amazon links, at no extra cost to you. This guide is based on manufacturer and listing information and general buying knowledge; we have not tested these products hands-on, and we do not list prices because they change often.
How a memory foam mattress is built
Most all-foam mattresses have at least two layers. The bottom is a dense support foam that keeps you from sinking too far. On top sit one or more comfort layers, often memory foam, which contour to your body and relieve pressure. Some add a transition layer between the two, or infusions such as gel, green tea or charcoal, which manufacturers use for cooling or freshness. We also reviewed the MLILY EGO 2 Inch Twin Mattress Topper if you want to compare.
The balance between support and comfort layers determines how the mattress feels. A thick comfort layer gives a deeper, softer feel. A thin comfort layer over firm support feels firmer and more on-top.
Hybrid mattresses combine foam comfort layers with a pocketed coil base. They tend to feel bouncier and sleep cooler than all-foam beds, but they are heavier and usually cost more. This guide focuses on all-foam, but the same firmness and size advice applies.
Choosing thickness
Five- and six-inch mattresses are best for children, bunk beds, trundles, daybeds and occasional guest use. They are light and affordable but may not offer enough support for adults who sleep on them every night, especially heavier adults.
Eight to ten inches suits most adults for everyday use and is the sweet spot for value. Twelve inches and above usually adds extra comfort layers for a plusher feel and can better suit heavier sleepers or those who want a deep, cushioned surface.
Check the height of your bed frame too. A thick mattress on a tall frame can make a bed awkward to climb into, while a thin mattress on a low platform may sit lower than you expect.
Firmness, sleep position and body weight
Firmness is often described on a 1 to 10 scale, where 10 is the firmest. Brand scales are not standardized, so use them as a rough guide and read reviews from people with a similar build and sleep position.
Side sleepers usually prefer medium-soft to medium, so the shoulders and hips can sink enough to keep the spine straight. Back sleepers often like medium to medium-firm. Stomach sleepers usually need firmer support to stop the hips sinking. Combination sleepers tend to do well on medium.
Body weight changes how a mattress feels. Lighter people may find a medium mattress feels firm, while heavier people may find the same mattress feels soft and may sink through thinner comfort layers. Heavier sleepers should look at thicker mattresses with denser support foam.
Cooling, smell and certifications
All-foam mattresses can feel warm because foam contours closely and restricts airflow. Gel infusions, open-cell foams and breathable covers can help, as can breathable bedding. If you sleep very hot, a hybrid may be worth considering.
Compressed mattresses arrive rolled in a box and need time to expand. Most are usable within a few hours, but full expansion can take a day or two, and there may be a temporary new-foam smell. Unbox it in a ventilated room.
Look for foam certifications such as CertiPUR-US, which indicates the foam has been independently tested for certain chemicals and emissions. Also check the listing for fire-safety information and the cover material, especially if anyone in the household has allergies.
Frames, foundations and warranties
Foam mattresses need a solid, supportive base. A platform bed, a slatted frame with slats no more than a few inches apart, or a metal platform frame all work. Old box springs designed for innerspring mattresses may not give foam even support. Check the mattress warranty, because using an unsuitable base can void it.
Metal platform frames are an affordable option that lift the mattress off the floor and often leave room for under-bed storage. Make sure the frame is rated for the combined weight of the mattress and sleepers.
Before you buy, read the warranty length and what it covers (sagging beyond a certain depth is a common term), and check the return policy for a mattress bought on Amazon, as it may differ from a manufacturer’s own trial.
Common mistakes to avoid
Buying too thin for nightly adult use to save money. A six-inch mattress that is fine for a guest room can feel unsupportive every night.
Trusting firmness numbers across brands. One brand’s medium can be another’s firm.
Putting a foam mattress on the floor long term. Without airflow, moisture can build up underneath. Use a frame or platform.
Judging the mattress on the first night. Your body needs time to adjust to a new surface; give it a few weeks unless something is clearly wrong.
Quick buying checklist
- Choose thickness for the use: eight to ten inches for nightly adult use.
- Match firmness to your sleep position and body weight, not just the brand’s number.
- Look for foam certifications such as CertiPUR-US.
- Plan for heat if you sleep warm: gel, open-cell foam or a hybrid.
- Make sure your frame or foundation gives solid, even support.
- Read the warranty terms and the Amazon return policy before ordering.
- Allow a day or two for full expansion and a few weeks to adjust.
How we choose our picks
The HashMonkey Company builds each shortlist from products that are popular in their Amazon category, then narrows it down using manufacturer specifications, the details on each listing and patterns in verified customer reviews. We look for clear information about materials and sizing, sellers with a solid track record and a good fit with the buying advice in this guide. We have not tested these products hands-on, so we do not claim one is objectively the best. Instead, each pick is matched to a specific need, so you can choose the one that fits how you will use it.
We do not accept payment for placement. We earn a commission through Amazon links, which helps keep the site running, but it does not change which products we include or how we describe them. If you spot an outdated detail, please let us know.
Our picks
These products are already on our shortlist. Each “Read more” link goes to our product page with details and who it suits; always confirm the size, specifications and seller on Amazon before you buy.
| Pick | Best for | Details |
|---|---|---|
| EGOHOME 8-Inch Memory Foam Mattress | Best everyday all-rounder | Read more · Check price on Amazon |
| ZINUS 8-Inch Cooling Essential Memory Foam Mattress | Best budget pick for warm sleepers | Read more · Check price on Amazon |
| FDW 8-Inch Twin Mattress | Best for kids’ rooms and guest beds | Read more · Check price on Amazon |
| ZINUS 5-Inch Green Tea Memory Foam Mattress | Best for bunk beds and daybeds | Read more · Check price on Amazon |
| New Jeto Metal Bed Frame | Best affordable platform frame | Read more · Check price on Amazon |
FAQ
How thick should a memory foam mattress be?
Eight to ten inches suits most adults for everyday use. Five to six inches is better for kids, bunk beds and occasional guest use.
Can I put a memory foam mattress on a box spring?
Only if the box spring gives firm, even support. Many foam mattresses work best on a platform or closely spaced slats. Check the warranty requirements.
How long does a mattress in a box take to expand?
Many are usable within hours, but full expansion can take a day or two.
Do memory foam mattresses sleep hot?
They can. Gel infusions, open-cell foams and breathable bedding help, and hybrids generally sleep cooler.
Which firmness is best for side sleepers?
Medium-soft to medium is usually most comfortable for side sleepers because it lets the shoulders and hips sink in.
Product details come from manufacturer and Amazon listing information as of October 2026 and may change. Always confirm the current price, specifications and seller on Amazon before you buy.