Active vs Total Coils: Sizing Solid Height and Travel
Short answer: Active coils (Na) are the free, deflecting coils that generate load; total coils (Nt) include the closed or ground end coils that only add height. For a typical closed-and-ground compression spring, Nt = Na + 2. Solid height is then approximately Nt × wire diameter (d), so a spring with 8 active coils and 1.0 mm wire has 10 total coils and a solid height near 10 mm. Usable travel is free length minus solid height, minus a safety margin of roughly 10–15% of deflection. If you compress past solid height, stress spikes and the spring takes a set.
Getting this distinction right is the difference between a spring that survives a million cycles and one that collapses in the first week of field use. Yet "number of coils" is one of the most ambiguously specified parameters on a drawing. A buyer writes "10 coils," the shop cuts 10 coils, and the spring arrives with only 8 that actually deflect — so the rate is wrong, the load is wrong, and the assembly either bottoms out or rattles.
This guide explains how active and total coils relate, how they determine solid height and maximum travel, and how to specify both unambiguously so your Dongguan spring supplier builds exactly what your mechanism needs.
What Are Active Coils in a Compression Spring?
Active coils are the turns that are free to deflect under axial load. They are the working portion of the spring — the part that stores and releases energy. Every load and rate calculation you do for a compression spring uses the active coil count, not the total.
The spring rate formula makes this explicit:
k = G·d⁴ / (8·D³·Na)
Where:
- k = spring rate (N/mm)
- G = shear modulus of the wire material (MPa)
- d = wire diameter (mm)
- D = mean coil diameter (mm)
- Na = number of active coils
Notice that Na sits in the denominator. Fewer active coils means a stiffer spring; more active coils means a softer one. If you double the active coil count while holding wire diameter and mean diameter constant, the rate halves. This is the single most powerful lever a designer has for tuning load without changing the wire or the envelope.
Why end coils don't count as active
On a closed-end compression spring, the two end coils are bent flat against the adjacent coil so the spring has a square, stable seating surface. Those coils cannot deflect independently — they are already in contact with the neighbouring turn. They contribute height and mass, but no travel and no rate. On a closed-and-ground spring, the ends are additionally ground flat to improve squareness and seating, but the coil count logic is the same: they are dead coils.
What Are Total Coils (and How Do They Differ)?
Total coils (Nt) is the literal count of wire turns from one end of the spring to the other. It includes the active coils plus the inactive end coils.
For the most common configurations:
| End condition | Relationship | Typical use |
|---|---|---|
| Closed, not ground | Nt = Na + 2 | General purpose, lower cost |
| Closed and ground | Nt = Na + 2 | Precision seating, high cycle life |
| Open, not ground | Nt = Na | Rare; spring seats on a flat washer |
| Open, ground | Nt = Na | Special applications only |
The "closed and ground" and "closed, not ground" cases both add two dead coils. This is why a spring drawing that says "10 coils total" and a calculation that assumes "10 active coils" will disagree by roughly 25% on rate — a gap large enough to fail a load test outright.
The relationship in one line
Nt = Na + number of dead end coils
If you remember nothing else from this article, remember that. Everything about solid height and travel flows from it.
How Do You Calculate Solid Height from Total Coils?
Solid height is the axial length of the spring when every coil is stacked metal-to-metal. It is the physical floor of the spring's travel — you cannot compress past it without yielding the wire.
The standard estimate is:
Solid height ≈ Nt × d
For a closed-and-ground spring, subtract a small allowance because grinding removes material from the end coils. A practical shop formula is:
Solid height = (Nt − 0.5) × d for ground ends
Solid height = Nt × d for unground ends
Worked example
Take a compression spring with:
- Wire diameter d = 1.2 mm
- Mean diameter D = 9.6 mm (spring index = 8)
- Active coils Na = 8
- Closed and ground ends
Then Nt = 8 + 2 = 10, and solid height ≈ (10 − 0.5) × 1.2 = 11.4 mm.
If the free length is 30 mm, the theoretical maximum deflection before solid is 30 − 11.4 = 18.6 mm. In practice you should never design to that limit. A working travel of 13–15 mm leaves a sensible margin and keeps stress below the material's allowable range.
| Parameter | Value | Notes |
|---|---|---|
| Wire diameter (d) | 1.2 mm | — |
| Mean diameter (D) | 9.6 mm | Spring index 8 |
| Active coils (Na) | 8 | Sets the rate |
| Total coils (Nt) | 10 | Na + 2 dead ends |
| Solid height | 11.4 mm | Ground ends |
| Free length | 30 mm | — |
| Theoretical max travel | 18.6 mm | To solid — do not use |
| Recommended working travel | 13–15 mm | ~75–80% of theoretical |
How Much Travel Can a Compression Spring Actually Use?
The gap between "theoretical travel to solid" and "safe working travel" is where most field failures are born. Three limits apply simultaneously, and the tightest one governs.
1. Solid height limit
You cannot exceed free length minus solid height. This is a hard geometric wall.
2. Stress limit
Even before solid, the wire stress at the inside of the coil may exceed the material's allowable torsional stress. For music wire and oil-tempered wire, a common design target is to keep working stress below roughly 40–45% of tensile strength for fatigue applications, and below about 60% for static or rarely cycled springs. These are indicative figures — the exact allowable depends on material, cycle count, and surface condition.
3. Set / scragging limit
New springs often lose a small amount of free length on first compression as residual stresses redistribute. Reputable manufacturers preset (scrag) springs that must hold free length tightly. If your design runs close to solid height, specify presetting explicitly on the drawing.
A practical rule
Design working deflection to no more than about 75–80% of the theoretical travel to solid. That single habit eliminates most solid-height-related failures without adding meaningful cost.
End Coil Configurations and Their Effect on Sizing
The end condition changes both the dead coil count and the solid height. It also changes how the spring seats, which matters for alignment and buckling.
| End type | Dead coils | Solid height | Seating quality | Relative cost |
|---|---|---|---|---|
| Closed, not ground | 2 | Nt × d | Good | Low |
| Closed and ground | 2 | (Nt − 0.5) × d | Excellent | Medium |
| Open, not ground | 0 | Nt × d | Poor (needs flat seat) | Low |
| Open, ground | 0 | (Nt − 0.5) × d | Fair | Medium |
For most industrial assemblies — valves, latches, connectors, appliance mechanisms, clamps — closed and ground ends are the default because they sit square and reduce side loading. Open ends are reserved for cases where the spring is captured on a rod or in a bore and squareness is not critical.
Sizing Travel: A Step-by-Step Method
Here is the sequence our engineers use when a customer sends a spring requirement with a target load and a known cavity.
1. Fix the envelope. Bore diameter and rod diameter set the maximum and minimum mean diameter, which sets the spring index (D/d). Aim for an index between 5 and 12; below 5 the spring is hard to wind, above 12 it tends to buckle.
2. Pick the material and wire diameter. Wire diameter drives both rate and stress. It is the coarsest adjustment.
3. Solve for active coils. Rearrange the rate formula: Na = G·d⁴ / (8·D³·k). Round to the nearest half coil.
4. Add dead coils. Nt = Na + 2 for closed ends.
5. Compute solid height. Use the ground or unground formula as appropriate.
6. Check travel. Free length minus solid height must exceed your required working deflection by at least 20–25%.
7. Check stress. Confirm the stress at maximum working deflection is inside the allowable band for your cycle life.
8. Specify the drawing. State Na, Nt, free length, solid height (max), rate, and load at one or two reference deflections.
Steps 3 and 4 are where the active/total confusion causes real damage. If you specify only one number, specify total coils, because that is what the coil winder counts — but always state the rate and a load at a reference height so the shop can verify the active count implicitly.
When to involve the supplier early
If your spring sits in a tight cavity, carries a dynamic load, or must hit a load window at two positions, send the assembly constraints rather than a finished spring spec. BQUQ quotes compression, extension, and torsion springs from Dongguan in 12 working hours, and flexible MOQ means a design iteration costs you days, not months. Early involvement usually collapses two or three prototype rounds into one.
Common Mistakes That Break Springs
Mistake 1: Specifying "coils" without saying which kind. Always write "active coils" or "total coils."
Mistake 2: Designing to solid height. Even a single over-travel event can set the spring permanently. Leave margin.
Mistake 3: Ignoring the end coil in rate calculations. This under-predicts stiffness by 20–30% on short springs.
Mistake 4: Forgetting presetting. If free length tolerance is tight and the spring runs near solid, specify scragging.
Mistake 5: Mixing up spring index and coil count. A spring can have many coils and still buckle if the index is high and the free length is long.
Mistake 6: Not verifying with a load test. A rate calculation is a prediction. A load-at-height measurement is a fact. See our notes on spring load testing for how to close that loop.
How BQUQ Controls Coil Count and Solid Height in Production
Coil count is set at the winding machine and then locked in by the heat-treat and preset steps. In our Dongguan factory, compression springs run on dedicated coiling lines with in-process checks on free length, outside diameter, and load at a reference height. Because CNC machining, metal stamping, custom springs, and heat sink production all sit under one ISO9001 roof, a spring that needs a stamped retainer or a machined seat can be developed as one package rather than three separate purchase orders.
Typical production tolerances for commercial compression springs are ±0.1 mm on outside diameter, ±1–2% on free length for preset springs, and ±10% on load at a reference height. Tighter windows are achievable but should be discussed at quoting, because they change the inspection plan and the scrap rate.
If your design is running into a stress wall — the spring is too stiff, or it yields before it reaches the required travel — the fix is often a material change rather than a geometry change. Our article on spring overload and yield walks through the diagnosis, and spring wire diameter selection covers how to trade wire size against coil count to escape a bad corner.
Frequently Asked Questions
Q: Is solid height calculated from active coils or total coils?
A: Always from total coils. Solid height is a physical stack-up of every turn of wire in the spring, including the dead end coils that never deflect. Using active coils under-predicts solid height by roughly two wire diameters, which means the spring will bottom out earlier than your drawing claims. Use Nt × d for unground ends, or (Nt − 0.5) × d for ground ends.
Q: How many active coils does a typical compression spring have?
A: Most industrial compression springs run between 4 and 12 active coils. Below about 4 the spring becomes very stiff and sensitive to end-condition variation; above roughly 12 it tends to buckle unless guided on a rod or in a bore. The exact count is set by the required rate, wire diameter, and mean diameter — not by convention.
Q: Can a spring have zero dead coils?
A: Yes, if both ends are open rather than closed. An open-end spring has Nt = Na, so every coil deflects. The trade-off is seating: open ends do not present a flat surface, so the spring must sit against a flat washer or inside a pocket. Open ends are common in torsion springs and in some extension spring designs.
Q: What happens if I compress a spring past its solid height?
A: The coils contact metal-to-metal and the spring becomes effectively rigid. Additional force goes into yielding the wire rather than deflecting the spring. The result is a permanent loss of free length — the spring "takes a set" — plus a stress spike at the end coils that can initiate a fatigue crack. Never design working travel to reach solid height.
Q: How do I specify coil count on a spring drawing?
A: State total coils and active coils separately, then add free length, solid height (as a maximum), rate, and load at one or two reference heights. The load-at-height figures let the supplier verify the active coil count indirectly, which catches winding errors before shipment. Ambiguous drawings that say only "10 coils" are the leading cause of spring rejection.
Related Resources
- About BQUQ — ISO9001 factory in Dongguan with four production lines under one roof
- Compression springs — coil, rate, and solid height specification support
- Extension custom springs — initial tension and body-length sizing
- Torsion springs — leg geometry and torque-per-degree calculations
- Technical articles — full library of spring and machining engineering guides
- Industry trends — sourcing and supply-chain notes for metal components
- Contact — send drawings for a quote in 12 working hours
Authored by the BQUQ Engineering Team. BQUQ (Dongguan) runs CNC machining (±0.005 mm), metal stamping, custom springs, and heat sink production in one ISO9001 factory. Source-direct from Dongguan, China — quote in 12 hours: sc@bquq.com | WhatsApp +86 13713157787 | www.bquq.com
