How do you calculate days of inventory coverage?

Divide on-hand inventory by daily usage to get raw days, then account for the safety-stock factor. In the example, 1,200 pieces at 85 per day gives about 14.1 raw days, reduced to 12.83 protected days after the safety cushion.

What is the difference between protected and unprotected days?

Unprotected days (14.12 here) is the naive inventory-divided-by-usage figure. Protected days (12.83) discounts for the safety multiplier, giving the coverage you can actually count on.

What is a good days-of-supply for rare earth magnets?

It should comfortably exceed your replenishment lead time. If magnets take 30 days to arrive, 12.83 protected days is well short of safe and you are exposed to a stockout.

How does lead time factor into coverage?

Lead time is the gap you must survive between reorder and receipt. Coverage below lead time means a line stoppage is likely if you reorder late, so target coverage above lead time plus buffer.

Why include a safety stock multiplier?

Demand and lead time vary. The safety factor trims the headline days to a conservative, dependable figure so you plan against a number you can trust, not a best case.

How do I raise my protected days of supply?

Increase on-hand inventory, reduce daily usage, or shorten lead time. Since single-source rare-earth lead times are hard to cut, buyers usually lift on-hand stock or qualify a second source.

Rare Earth Magnet & Motor Materials calculator

Inventory Coverage Calculator

Inventory coverage tells a magnet or motor-materials buyer how many days of production their on-hand stock of rare earth magnets will actually protect, given daily consumption, supplier lead time, and a safety-stock cushion. Supply-chain planners rely on it because rare-earth sourcing is concentrated and lead times can swing hard, so running short means idling a motor assembly line waiting on offshore replenishment. It reframes a pile of inventory into the metric that matters — protected days of supply — and contrasts that against the lead-time exposure you are trying to cover. That comparison is what tells you whether your buffer is genuinely safe or dangerously thin.

What this calculator does

Estimate inventory coverage for rare earth magnet and motor materials using production-ready inputs so teams can plan replenishment and safety stock using actual usage and lead time.
Use it when inventory coverage in rare earth magnet and motor materials is being sized for a buffer or safety stock review.
It converts on-hand magnet stock into protected days of supply using daily usage, lead time, and a safety-stock factor.

Formula used

Inventory coverage cycle stock = inventory coverage daily usage × inventory coverage lead time
Required inventory coverage inventory = cycle stock + inventory coverage safety stock

Inputs explained

Magnet consumption per day:
Supplier replenishment lead time:
Safety stock multiplier:

How to use the result

Use it when setting reorder points or reviewing exposure on long-lead, single-source rare earth magnet inventory.
It assumes steady daily usage; a demand spike or a stretched lead time erodes the protected days faster than the static number implies.

Current U.S. benchmarks

The producer price index for copper and brass mill shapes stands at 559.593 (BLS, May 2026), up 76.8% from a year earlier. Quotes priced off last quarter's material cost miss this move. Global copper trades at $13,484 per tonne (IMF via FRED, May 2026).
The U.S. has 5,397 electrical equipment and appliances establishments employing about 369,437 workers (Census County Business Patterns, 2023).

Common questions

How do you calculate days of inventory coverage? Divide on-hand inventory by daily usage to get raw days, then account for the safety-stock factor. In the example, 1,200 pieces at 85 per day gives about 14.1 raw days, reduced to 12.83 protected days after the safety cushion.
What is the difference between protected and unprotected days? Unprotected days (14.12 here) is the naive inventory-divided-by-usage figure. Protected days (12.83) discounts for the safety multiplier, giving the coverage you can actually count on.
What is a good days-of-supply for rare earth magnets? It should comfortably exceed your replenishment lead time. If magnets take 30 days to arrive, 12.83 protected days is well short of safe and you are exposed to a stockout.
How does lead time factor into coverage? Lead time is the gap you must survive between reorder and receipt. Coverage below lead time means a line stoppage is likely if you reorder late, so target coverage above lead time plus buffer.
Why include a safety stock multiplier? Demand and lead time vary. The safety factor trims the headline days to a conservative, dependable figure so you plan against a number you can trust, not a best case.

Last reviewed 2026-05-12.