A reader emailed to ask why we rate the Helix Midnight Luxe a 5.5 when two other sites call it a 6.5 and the manufacturer says "medium". It is a fair question and the answer is more interesting than "they are wrong".
We pulled published firmness ratings for six mattresses across eight review sites, plus the manufacturer's own figure.
The four reasons
1. Nobody measures the same way
Of the eight sites, three publish a method. One uses a compression gauge like ours, one uses a weighted plate at a fixed depth, and one describes "consensus of our review team". The other five do not say.
A gauge presses a small plate into a single point. A person distributes their weight over several square feet and sinks further. These produce different numbers on the same mattress and both are defensible.
2. Tester weight is the dominant variable and is rarely stated
This is most of the spread. Firmness is a relationship between a surface and a load, and a mattress that reads 5.5 under 175 lb reads closer to 4 under 240 lb — the comfort layer bottoms out and you are further into the support core.
Only two of the eight sites publish their tester's weight. A firmness rating without a weight attached is an incomplete statement, in the same way that a temperature reading without a location is.
3. Sample variation is real
We have measured two units of the same model, bought four months apart, at 5.5 and 6.0 on the same gauge. Foam is a chemical product with batch variation, and a quarter-point of manufacturing spread is normal.
That is small next to the tester-weight effect, but it means two honest sites measuring carefully can legitimately publish different numbers.
4. Some of it is the manufacturer
Brands describe their own mattresses generously and inconsistently. "Medium-firm" is used for surfaces we have measured between 5 and 7.5. There is no standard and no certifying body, so the word means whatever the marketing department needs it to mean.
What we do about it
Three things, all of which are in the how we test page:
- The gauge reading is sampled at nine points, not one, because mattresses are not uniform and edge readings run firmer than centre readings.
- Every rating is cross-checked against three testers at 135, 175 and 240 lb, and where they disagree we publish the disagreement rather than averaging it away.
- Every review lists a weight range in its "look elsewhere if" section, because that is the information the number is missing.
How to read someone else's rating
Ask two questions. What was the method, and who was lying on it? If a site answers neither, the number is a vibe rather than a measurement — which does not make it useless, but it does mean you should treat a 6.5 and a 5.5 from two such sites as the same claim.
And whatever the number says, the firmness guide rule still applies: under 150 lb, buy softer than the reviews suggest; over 230 lb, buy firmer.