Why days of cover instead of a safety multiplier?

A multiplier of 1.1 is hard to audit and hides what it adds. Days of cover is observable: 70 days of lead time plus 7 safety days gives 77 days, and the two numbers can be checked.

When is required stock a poor guide?

When failures cluster. One pump failure removes a fixed unit, so two failures in a week hurt more than the same average spread across a year. Track failure events, not only average usage.

Should the cap include stock on order?

Enter only units you can physically store. Stock already on order belongs with lead-time demand, and counting it twice would understate the next order point, so keep the cap tied to shelf space.

How do I choose safety stock days?

Start from the longest recent delivery delay plus the repair time for one pump. If a stockout would stop a treatment process, add the days your team needs to source an emergency replacement.

Water, Wastewater & Pump Systems Manufacturing calculator

Spare Pump Lead Time Calculator

Set spare stock from daily consumption, the replenishment lead time and the safety days you want, then compare it with the space you have. Days of cover is simply lead time plus safety days.

What this calculator does

Spare pumps to hold for a supplier lead time and safety cover, checked against the maximum you can store.

Formula used

Required stock units = spare pumps per day × (replenishment lead time days + safety stock days)
Lead-time demand units = spare pumps per day × replenishment lead time days
Safety stock units = spare pumps per day × safety stock days
Days of cover = required stock units ÷ spare pumps per day
Stock headroom units = maximum stock on hand − required stock units

Inputs explained

Spare Pumps Consumed per Day: Average pumps drawn from stock each day, from issue records.
Supplier Replenishment Lead Time: Days from placing a supplier order to receipt at the crib.
Safety Stock Days of Cover: Extra days of cover held against delays and failure clusters.
Maximum Stock on Hand: Most units the crib can store, from the rack layout.

How to use the result

Best suited to sizing a spares crib, reviewing a supplier lead time, planning around a storage cap.
Failure-driven demand is lumpy, so one large capital pump order can exceed the average this model assumes. A binding stock cap forces a choice between expediting, a second supplier and accepting stockout risk.

Current U.S. benchmarks

The U.S. has 21,668 machinery manufacturing establishments employing about 1,086,146 workers (Census County Business Patterns, 2023).

Common questions

Why days of cover instead of a safety multiplier? A multiplier of 1.1 is hard to audit and hides what it adds. Days of cover is observable: 70 days of lead time plus 7 safety days gives 77 days, and the two numbers can be checked.
When is required stock a poor guide? When failures cluster. One pump failure removes a fixed unit, so two failures in a week hurt more than the same average spread across a year. Track failure events, not only average usage.
Should the cap include stock on order? Enter only units you can physically store. Stock already on order belongs with lead-time demand, and counting it twice would understate the next order point, so keep the cap tied to shelf space.
How do I choose safety stock days? Start from the longest recent delivery delay plus the repair time for one pump. If a stockout would stop a treatment process, add the days your team needs to source an emergency replacement.

Last reviewed 2026-10-01.