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The best ways to make a soft mattress firmer are by adding a firm mattress topper, placing a plywood board between your mattress and foundation, or adjusting your bed's support system.
A too-soft mattress creates an unsupported sleeping surface that can cause back pain, disrupt sleep quality, and make getting in and out of bed difficult.
The 15 tips below show you how to make a mattress harder, including modifying your existing mattress, adjusting your bed's foundation, and environmental changes that affect mattress firmness.
A firm mattress topper makes a soft mattress firmer as soon as you lay it on the bed. The fix works on any mattress type (innerspring, memory foam, latex, or hybrid).
Place the topper on top of your mattress and secure it with your fitted sheet. There is nothing to cut, screw, or assemble.
A mattress topper makes a bed firmer by adding extra thickness to the top of your mattress.
If you choose a mattress topper rated firm or extra firm that is 3-4 inches (7.62 to 10.16 cm) thick, it is enough to completely transform the feel of your bed.
The 4" PlushBeds Quilted Organic Mattress Topper in firm (40-42ILD) feel is the most effective option to stiffen up a soft mattress without replacing it.
Other firm options are listed in our organic mattress toppers guide.
A queen-size certified organic topper starts at around $250, which is more expensive than most solutions on this list, but significantly cheaper than the cost of a new mattress.
Insert a 3/4-inch (1.9 cm) plywood board between your mattress and box spring or bed frame to stiffen up your bed foundation.
Cut the plywood slightly smaller than your mattress dimensions for easier handling. Sand all edges to prevent splinters and damage to your mattress.
Placing plywood under a mattress makes a bed feel firmer by preventing the mattress from sinking into the box spring or slats, which creates a completely rigid base.
Plywood costs under $50, works immediately, and is especially effective at firming up innerspring mattresses.
If you use a board, perforate it and cover it with fabric to help with ventilation. Ventilating plywood is important because body heat normally dissipates through the mattress, and it could become trapped without ventilation.
Use untreated plywood to avoid chemical exposure. I recommend you do not use MDF because it contains formaldehyde.
Replace your soft mattress with a firm eco-friendly model if the other fixes on this list don't give you enough improvement.
Choose a natural latex mattress labeled firm or extra firm. The certified firm models in our eco-friendly mattress guide come from brands such as PlushBeds, Avocado, and Naturepedic.
Replace your mattress instead of firming it up when the mattress itself has worn out.
The clearest signs that a mattress needs replacing are a sag that does not spring back, new aches when you wake, and sleeping better in other beds.
Check your sleep trial and warranty terms before you spend money on any other fix.
If your mattress is new, you may still be inside the sleep trial, which lets you return it and choose a firmer model for free. If the trial has ended, check whether the warranty covers deep body impressions.
A sag usually has to reach 1 inch (2.5 cm) to 1.5 inches (3.8 cm) deep before a manufacturer accepts a warranty claim.
Measure the depth by laying a straight edge across the mattress and measuring the gap beneath it. Photograph the measurement and send it to the manufacturer with your proof of purchase.
Rotate your mattress 180 degrees every 3 months. I recommend marking your mattress with seasonal indicators (like "Spring," "Summer") so you always know when the next rotation is due.
Rotating a mattress makes it feel firmer by moving your body onto foam that has carried less weight and kept more of its support. Regular turns help spread wear evenly, which stops a soft spot from deepening under your usual sleeping position.
Rotating means turning the mattress 180 degrees so the head end swaps with the foot end. Flipping means turning the mattress over so you sleep on the other side.
Rotation delivers modest benefits on any mattress, but flipping only works on double-sided models.
If you have a double-sided mattress, flip it so the less compressed side faces up. Flip a double-sided mattress every 6 months and rotate it 180 degrees in between, so both sides wear evenly.
A side that has carried less body weight keeps more of its original firmness, so flipping it makes the bed feel firmer right away. The longer the mattress has gone without flipping, the bigger the difference you feel.
No, most modern mattresses are one-sided. You should not flip a one-sided mattress, because the underside is not designed to be slept on.
If you have a pillow top or one-sided memory foam mattress, use one of the other fixes on this list, such as a firm topper.
Replace your bed base with a platform bed or a solid foundation. Choose a platform with a solid surface, or slats no more than 3 inches (7.6 cm) apart.
A solid foundation makes a mattress firmer because it stops the bed base from flexing under the mattress.
A worn base or an old box spring flexes under your weight, and that flex makes the bed feel softer. A solid foundation does not flex, so you feel the full firmness of the mattress.
Yes, you can fix a slatted foundation by adding extra slats or fitting support bars under the frame.
Fix 13 below explains the slat spacing to aim for. Adding slats costs $20 to $60, while replacing the whole foundation costs $200 to $500.
If you use a box spring, replace it if it is more than 7 years old or shows signs of sagging. Choose a replacement with rigid construction and closely spaced internal supports.
A worn box spring makes a mattress feel softer when its coils have lost tension and no longer push back against your weight. The mattress sinks into the sagging coils beneath it, and you feel that as a soft spot in the bed.
No, most modern mattresses do not need a box spring; only older innerspring mattresses are designed to use one. Foam, latex, and hybrid mattresses need a platform or a solid foundation.
Lower your bedroom temperature at night to make a memory foam mattress firmer. Use a programmable thermostat to drop the temperature before bed, or open a window during the cooler months.
A cooler room makes a memory foam mattress firmer because memory foam stiffens as it cools and softens as it warms (Malewska et al., 2026). Temperature sensitivity is why cool temperatures only work to firm up memory foam mattresses, and not any other material.
Your bedroom should be between 65°F (18°C) and 68°F (20°C) for a firmer mattress, cool enough to firm the foam without being too cold for comfortable sleep.
Move your mattress off its bed frame and lay it directly on the floor to test whether a firmer base solves the problem.
The floor is the firmest base a mattress can sit on because frames, foundations, and box springs all flex a little under body weight, and a solid floor does not.
No, sleeping with your mattress on the floor long-term can void your warranty and trap moisture.
This is a quick and cheap way to test whether the extra firmness helps your sleep. If it does, buy a firmer foundation and move the mattress off the floor again when you can.
Strip off all the bedding and let your mattress breathe for 4 to 6 hours in a well-ventilated room. When the weather allows, stand the mattress outside in direct sunlight, which dries it faster.
Airing out a mattress makes it firmer by drying out any moisture that has soaked into the foam and softened it (Yu et al., 2011).
Expect a modest improvement, mainly on foam mattresses in humid bedrooms. Airing does not fix permanently compressed foam.
Air out your mattress every 3 months, or more often if your bedroom is humid or you sweat heavily at night.
Slide a mattress support between the mattress and the foundation, directly beneath the dip. Supports cost $20 to $100.
An under-mattress support fixes a sagging area by pushing the dip back up from below. Mattress supports don't fix worn foam, but they do take the weight that the foam no longer can.
Yes, supports like Mattress Helper work on a localized dip, but not on a mattress that is soft all over or severely sagged. A mattress that has gone soft across the whole surface needs a whole-bed fix, such as a firm topper.
If you have a severe sag, follow the steps in our guide to fixing mattress indentation for best results.
Add more slats if you have a slatted bed frame, or replace the whole set, so the slats sit closer together.
Closer bed slats make a mattress firmer by stopping it from sinking into the gaps between them. Wide gaps between slats allow the mattress to dip a little under your weight, and those dips add up to a softer bed.
Bed slats should be no more than 3 inches (7.6 cm) apart. Choose hardwood slats where you can, because they flex less than softwood. If you want no gaps at all, adding a bunkie board (fix 14) or a solid platform (fix 7) is the easiest solution.
Lay the board on your existing base and put the mattress on top. A bunkie board is a thin, solid foundation, 1 inch (2.5 cm) to 3 inches (7.6 cm) thick, made to sit between a mattress and a bed frame or box spring.
Bunkie boards cost $50 to $150 on Amazon, depending on size.
A bunkie board makes a mattress firmer the same way a solid foundation does (fix 7), by giving the mattress a rigid surface that does not flex. The board sits on top of the frame or box spring, so you get a solid base without replacing the bed.
A bunkie board is better than plywood when you want a ready-made board that fits your mattress, but it costs more. A bunkie board comes cut to size and covered in fabric, whereas plywood needs cutting, sanding, and perforating first (fix 2).
Unzip and remove the pillow top layer if your mattress has a removable one, which exposes the firmer core underneath. The fix costs nothing and works right away.
Removing a pillow top makes a mattress firmer because the pillow top is a layer of soft padding on top of a firmer support core. Without that padding, you sleep directly on the core, so the bed feels firmer but less cushioned.
The Naturepedic EOS lets you change the firmness without buying a new mattress, because you can unzip it and swap or rearrange the layers inside.
The EOS Classic comes in five firmness levels up to extra firm. You can set each side of the bed differently, and layer swaps are free during the 100-night trial.
The EOS Pillow Top, which I tested for our guide to the best organic mattresses for side sleepers, includes a zippered pillow top compartment that swaps out like the other layers.
Mattress firmness is important because it determines whether your spine keeps its natural shape while you sleep.
If you find your mattress is too firm, follow our guide to making a firm mattress more comfortable.
A firm mattress is better for your back than one that is too soft, but a medium-firm mattress is better than either (Caggiari et al., 2021, Kovacs et al., 2003).
Medium-firm is therefore the ideal level for an organic mattress for back pain.
But every sleeper is different, and there are other factors to consider when choosing your ideal firmness level.
To choose the firmness level you need, consider your sleeping position, body weight, and personal preferences.
The table below summarizes recommended mattress firmness levels based on your sleep position and body weight.
| Sleep Position | Body Weight | Recommended Firmness | Benefits |
|---|---|---|---|
| Side Sleeper | Under 130 lbs (59 kg) | Soft to Medium-Soft (3-4/10) | Cushions shoulders and hips, prevents pressure points |
| Side Sleeper | 130-230 lbs (59-104 kg) | Medium (5-6/10) | Balances support with pressure relief at hip and shoulder |
| Side Sleeper | Over 230 lbs (104 kg) | Medium-Firm (6-7/10) | Prevents excessive sinking while cushioning pressure points |
| Back Sleeper | Under 130 lbs (59 kg) | Medium-Soft to Medium (4-5/10) | Supports lower back while allowing slight contouring |
| Back Sleeper | 130-230 lbs (59-104 kg) | Medium-Firm (6-7/10) | Maintains spinal alignment with adequate lumbar support |
| Back Sleeper | Over 230 lbs (104 kg) | Firm (7-8/10) | Prevents excessive sinking that could misalign spine |
| Stomach Sleeper | Under 130 lbs (59 kg) | Medium (5-6/10) | Prevents pelvis from sinking while comfortable for chest |
| Stomach Sleeper | 130-230 lbs (59-104 kg) | Medium-Firm to Firm (6-8/10) | Keeps spine aligned by preventing midsection from sinking |
| Stomach Sleeper | Over 230 lbs (104 kg) | Firm to Extra-Firm (8-10/10) | Provides maximum support to maintain neutral spine position |
Test your chosen firmness for at least 30 nights to allow your body to adjust. Many mattress companies offer sleep trials of 100 nights or more, giving you ample time to determine if the firmness level meets your needs.
Your mattress is too soft when your hips sink lower than your shoulders as you lie on your back.
A too-soft mattress leaves you with stiffness or lower back pain in the morning that eases once you are up and moving. A soft mattress lets you sink so far that rolling over takes effort.
If you sleep better on a firmer bed away from home, your own mattress is too soft.
Making a mattress firmer costs anywhere from nothing to $5,000, depending on the cause of the softness and the fix you choose.
Rotating or flipping the mattress, moving it to the floor, and airing it out cost nothing. Plywood costs under $50. A firm organic topper starts at around $250. A new eco-friendly mattress costs between $689 and $5,000, depending on the brand, size, and specification you choose.
A mattress becomes soft because foam compresses permanently under your body weight and coils lose their tension over years of use.
Foam and coils wear fastest in the spots where you sleep every night, which is why body impressions form before the whole mattress softens.
Moisture absorbed from sweat and humid air also softens mattress foam. Low-density foam softens faster than dense foam or latex, so a cheap mattress goes soft years before a quality latex one, as our guide to how long a mattress lasts explains.
No, a mattress pad cannot make a mattress noticeably firmer.
A mattress pad is a thin quilted layer made to protect the mattress and add light cushioning. A thin pad cannot change how far you sink into the foam or coils beneath it.
A firm mattress topper of 3 inches (7.6 cm) or more is the best option to create a firmer surface, as Fix 1 above explains.
Yes, there are some risks as listed below.
