The coils, also known as the core or the innerspring unit, provide the main support for the body. Proper support is essential not only for a good night’s sleep, but for maintaining a healthy spine. When considering the core of an innerspring mattress, there are a number of important factors: the number of coils used throughout the mattress, the way the coils are constructed, and the shape of the coils.
You might have heard a rumor that the more coils in a mattress core, the more supportive the mattress will be. While this can be true, coil count is not nearly as critical today as it once was. These days the construction of the coils is a much more important factor in determining the overall comfort and support of the mattress. That being said, here are some guidelines to follow vis-à-vis coil count:
- A typical mattress contains between 250 and 1,000 coil springs
- A mattress with a lower coil count might use thicker wire or other techniques to compensate for this (see Coil Construction below)
- As a rule of thumb, the minimum number of coils you should look for in a mattress is 300 for a full, 375 for a queen and 450 for a king-size mattress
When discussing the coils of an innerspring mattress, the word “gauge” refers to the thickness of the wire used to construct the coil. Somewhat counter-intuitively, the lower the number, the “heavier” the gauge, and the thicker the wire — 12.5 gauge wire (very “stiff”) is thicker and stronger than 16.5 gauge wire (quite “springy”).
Another important factor is the number of “working turns” in each coil, a measurement of how tightly the coil is wound. Coils constructed with a higher number of working turns will result in both a softer and more durable mattress, since the work of supporting your body is spread throughout the spring. Working turns can be counted by tracing the wire with your finger — each time your finger travels 180 degrees around the coil translates to one working turn.
Tempering is the process of heating and cooling the coils to ensure they retain their proper shape over time. A “double heat tempered” coil has gone through that process twice, ensuring increased durability.
There are three main coil designs used in modern mattresses: Hourglass, Pocketed and Continuous.
Hourglass coils, the most commonly used design, come in two varieties — Bonnell coils and Offset coils. Based on 19th century buggy seat springs, the Bonnell coil has an hourglass shape with a knot at each end, and is known as the original mattress coil. Today, Bonnell coils are still the most prevalent coils in the mattress industry, though they are typically found in less expensive mattresses. The Offset design, found in more expensive mattresses, is similarly hourglass shaped, but the circles at the top and bottom of each coil are flattened to create a hinging action within the mattress core. This design allows the mattress to better conform to your body, and tends to make Offset coils less noisy than their Bonnell predecessors. It also allows Offset coils to be connected to each other via helicals (corkscrew-shaped wires that run over the tops and bottoms of the coils), which prevents them from moving from side to side when compressed.
Pocketed coils, also known as Marshall coils, are each individually wrapped in a fabric pocket. While the pockets might be connected together, the springs are each independent of one another and can move separately. Pocketed coils provide more motion separation than other innerspring coil types, meaning that when your sweetie starts tossing and turning, you are less likely to be disrupted from your zzz’s. This can really come in handy if your bed-mate is a restless sleeper. On the flip side, pocketed coils endure greater strain over time, since each coil absorbs weight without distributing it to other nearby coils. Simmons uses pocketed coils in most of their innerspring mattresses.
Continuous coil springs are made from a single length of wire shaped into a series of loose S-shaped ringlets. The concept behind this design is that by attaching each coil to its neighbors, the mattress core will be stronger, more stable, and more durable. Continuous coil designs also allow for significantly more coils per mattress, making it difficult to compare coil counts with non-continuous coil systems. Both Serta and Kingsdown tend to use continuous coils in their products.