Most people spend around £500–£1,500 on a new mattress in the UK, yet very few understand what they're actually buying. The spring system — often invisible, rarely discussed — determines roughly 60–70% of how a mattress performs over its lifetime. Get it wrong and you're looking at back pain, broken sleep, and a mattress that sags within three years. Get it right and the same bed could comfortably last a decade or more.
Here's the honest breakdown.
What Are Mattress Coils, and Why Do They Matter?
Coils are the structural backbone of innerspring and hybrid mattresses — the two most popular mattress types in the UK, together accounting for over 65% of the market according to trade data from The Sleep Council. They're made from tempered steel wire, a process of repeated heating and cooling that increases resilience, helping the spring return to its original shape even after years of compression.
The job of the coil system is straightforward: support your body weight, distribute pressure evenly, and keep your spine aligned. But beyond that basic function, the type of coil used has enormous consequences for motion isolation, durability, noise, temperature regulation, and how well the mattress moulds to different body shapes.
Coil-based mattresses also sleep notably cooler than all-foam alternatives. The open structure creates natural air channels throughout the support core — a meaningful advantage given that the average person loses roughly half a litre of sweat per night.
The Four Main Coil Types
1. Bonnell Coils — The Original, Still Going Strong
Bonnell coils are the oldest spring design in mattress manufacturing, dating back to the 1860s when they were adapted from buggy seat springs. The shape is an hourglass: wider at the top and bottom, narrower in the middle. This graduated structure means the spring responds progressively — lighter pressure engages the thinner middle section, while heavier loads compress the wider base.
Each coil is knotted at both ends and laced together with helical (spiral) wires, forming a single interconnected unit.
The trade-off: because everything is joined, when one part of the mattress moves, the rest feels it too. This is called motion transfer, and it's the primary complaint from couples sharing a Bonnell spring bed. A 2019 consumer review analysis by Which? found that motion transfer was among the top three complaints for budget innerspring mattresses — most of which use Bonnell systems.
Coil count range: 400–600 for a UK king size
Typical gauge: 12.5–14 (firmer, more durable)
Price point: Entry-level (£200–£600 for a king)
Best for: Solo sleepers, guest beds, those on a tighter budget
2. Offset Coils — A Meaningful Upgrade
Offset coils start with the same hourglass foundation as Bonnell, but the tops and bottoms are flattened into squared-off sections. These flat faces connect via helical wires at hinge points, allowing each coil to flex independently before the tension spreads to adjacent springs. Think of it as a hinge mechanism rather than a rigid joint.
The practical result: better body contouring, reduced noise, and lower motion transfer than Bonnell coils — without the cost premium of pocketed springs.
Several offset variations exist. Hinged offset coils have flex points on both the top and bottom. Knotless offset designs reduce friction between springs, which further cuts down on noise — a benefit for light sleepers.
Coil count range: 500–2,000 (the higher end uses mini-offset coils)
Typical gauge: 13.5–15 (medium firmness with good flex)
Price point: Mid-range (£500–£1,200 for a king)
Best for: Couples who want better motion control without going fully hybrid
3. Continuous Wire Coils — Underrated Durability
Continuous wire coils are made from a single, unbroken strand of steel wire, looped into row after row of connected springs across the entire mattress. Helical wires run between rows to maintain spacing.
This design is structurally very strong. Because the wire runs from one side of the mattress to the other without any joins, there are fewer weak points where the structure can break down. Continuous wire mattresses often retain their shape well over time — particularly useful for heavier sleepers.
The limitation is contouring. A single wire running horizontally across the mattress is inherently less responsive to localised pressure than individually working coils. Side sleepers and those with pronounced hip-to-shoulder differences may find the surface too uniform. That said, paired with a generous comfort layer — 5cm or more of foam or latex — a continuous wire base performs well.
Coil count range: 400–800 for a UK king size
Typical gauge: 13–15
Price point: Budget to mid-range (£200–£800)
Best for: Front sleepers, back sleepers, those who prioritise long-term durability
4. Pocketed Coils — The Current Industry Standard
Pocketed coils (also called Marshall coils in the UK) are individually wrapped in fabric — typically a non-woven polyester. Each spring sits inside its own sleeve, which is then glued or sewn to its neighbours. This means every coil compresses independently in response to direct pressure, with no mechanical link forcing it to interact with adjacent springs.
The implications are significant:
Motion isolation: Independent movement means one side of the bed can shift without disturbing the other. Research from the National Sleep Foundation (US, but the sleep biology applies universally) found that couples sharing a bed report 30% fewer sleep disruptions on pocketed coil mattresses compared to connected spring systems.
Contouring: Because each spring responds to its precise load, pocketed coils adapt more accurately to body shape — particularly along the shoulder, waist, and hip curve. This is why pocketed coils dominate the hybrid mattress category, where the goal is to combine targeted spring support with foam comfort.
Durability: The cylindrical shape distributes pressure across the full height of the coil rather than concentrating it at the midpoint like an hourglass spring. This structural advantage means pocketed coils typically show less central sagging over time.
Coil count range: 800–3,000+ for a UK king size (some premium models reach this through micro-spring layers)
Typical gauge: 14–16 (finer, more responsive wire)
Price point: Mid to premium (£700–£2,500+)
Best for: Couples, side sleepers, those with joint pain, anyone who runs hot
Microcoils, Minicoils, and Nanocoils: The Comfort Layer Revolution
Standard innerspring coils are around 15cm tall. Microcoils measure 2.5–7.5cm, minicoils fall in a similar range, and nanocoils can be under 2.5cm — some barely exceeding 1cm in height.
These aren't support springs. They live in the comfort or transitional layers above the main coil unit, typically found in higher-end hybrid mattresses from brands like Saatva, Emma, and Hypnos. Their job is to provide pressure relief and support simultaneously — bridging the gap between the firm base below and the sleeper above.
Because they're individually pocketed in virtually every application, microcoils add motion isolation to the comfort layer itself. They also maintain airflow throughout a section of the mattress that foam alone would trap heat in.
Some luxury hybrids stack two distinct coil systems: a pocketed support core underneath and a microcoil comfort layer on top, separated by a thin foam transition. This architecture tends to command prices above £1,500 for a king, but the combination of support, pressure relief, and temperature neutrality is difficult to match.
Coil Count and Gauge: What to Actually Pay Attention To
These two numbers are frequently used in marketing, and frequently misunderstood.
Gauge refers to wire thickness. In the UK, mattress coils typically run 12.5 to 17 gauge. Counterintuitively, a lower gauge number means thicker wire. Thicker = firmer and more durable. Thinner = more responsive and flexible. A 12.5-gauge coil will feel significantly firmer underfoot than a 16-gauge coil made from the same steel.
Coil count is the total number of springs in the mattress. Rough minimums to look for in a UK king size (150cm x 200cm):
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Bonnell/continuous wire: no fewer than 400
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Offset: no fewer than 500
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Pocketed: no fewer than 800 (some well-regarded models go above 2,000)
However — and this is important — a high coil count is not automatically better. A mattress with 3,000 low-quality, untreated steel coils will underperform a mattress with 1,000 tempered, high-grade springs. What matters more: whether the steel is tempered, the gauge consistency across the mattress, and the number of turns per coil (more turns = more flex range).
One practical indicator: if a manufacturer leads with coil count as a headline selling point and doesn't mention gauge or tempering, treat that as a red flag.
Which Coil Type Is Right for You?
You're on a budget, sleep alone: Bonnell or continuous wire. Look for gauge 13–14 and a coil count above 400 for a king. Prioritise a thicker comfort layer to compensate for lower contouring.
You share a bed and sleep lightly: Pocketed coils are non-negotiable. Motion transfer from connected systems is genuinely disruptive to sleep quality. Aim for 1,000+ individually wrapped coils in a king.
You're a side sleeper: Pocketed coils with a softer gauge (15–16) or a hybrid with a microcoil comfort layer. Side sleeping puts asymmetric pressure on the shoulder and hip — you need a spring that compresses locally, not one that redistributes force laterally.
You're over 16 stone (100kg+): Continuous wire or low-gauge (12.5–13.5) pocketed coils. Structural integrity under sustained heavy loads matters more than fine contouring. Check that the edge support is reinforced — often done with a border rod or perimeter coils of a lower gauge.
You run hot at night: Any coil system beats foam-only. If temperature is a significant concern, a hybrid with pocketed coils and no memory foam comfort layer is your best option. Memory foam traps heat by design; latex or wool comfort layers over a coil base are far more breathable.
The Bottom Line
The spring system isn't a detail — it's the foundation everything else rests on. A premium comfort layer on top of a poorly engineered coil unit is a short-term fix; within two to three years, the inadequate base will show. Conversely, a well-built pocketed coil system paired with a modest comfort layer can outperform a flashy mattress with a substandard spring unit by a significant margin.
When you're next shopping for a mattress, ask the retailer — or check the spec sheet — for coil type, gauge, and whether the steel is tempered. If they can't answer those three questions, look elsewhere.
Reference
Author
Jayant Upadhyay is a health writer and content strategist with 13+ years of experience in SEO-driven content and research-led publishing. He has created 5,000+ articles across health, wellness, and lifestyle, focusing on evidence-based insights that improve sleep, well-being, and everyday health outcomes for global audiences.
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