How to calculate the cost of unplanned downtime

Everyone on the floor agrees unplanned downtime is expensive. But just how expensive is it? Depending on the situation it can range from a few hundred dollars an hour to over $250,000, but how do we figure out your number?

This guide gives you a cost-per-hour formula you can operate with. It breaks the number into four parts — idle labor, lost margin, startup scrap, and absorbed overhead — shows a full worked example from an injection molding cell, and is honest about which terms are contested so you know which ones to defend.

Why one number never fits

A calculated cost of downtime is untrusted because they blend costs that behave very differently. Some are cash you spend whether the machine runs or not. Some are money you never earn because parts did not get made. Bundling them into a single average hides the assumptions, and assumptions are exactly what a skeptical business owner will attack.

So build the number from components. Each component answers a different question, and you can decide for your own plant which ones belong. The formula is:

Cost per downtime hour = idle labor + lost contribution margin + startup scrap + absorbed fixed overhead

Work through each term before you add them up.

Term 1: idle labor

When a machine stops unexpectedly, the operators standing next to it are usually still on the clock. That wage is a real, out-of-pocket cost of the stop.

Two rules keep this honest. First, use the fully loaded labor rate — wage plus payroll tax, benefits, and shift premium — not the base hourly wage. The fully loaded rate is typically 1.25 to 1.4 times base pay. Second, only count labor you genuinely cannot redeploy. If your operators run to another cell or do 5S during a long stop, that time is not lost, and counting it overstates the cost. For short unplanned stops, though, nobody redeploys, so the full crew wage counts.

Term 2: lost contribution margin

This is the biggest and most-argued term, and it is where most homegrown estimates go wrong. When the line is down, it does not make parts, and each part not made is margin you never earn.

The critical word is margin, not price. Use the contribution margin per part — selling price minus the variable cost of making it (material, and any variable energy or consumables). Do not use the full selling price; the material you did not consume is money you did not spend, so counting the whole price double-counts. And do not use gross profit that already subtracts fixed overhead — you will handle overhead separately in term four.

There is one condition on this term: it only applies if the line is capacity-constrained. If the cell is a bottleneck running flat out and every lost hour is an order you cannot ship, the lost margin is real. If the cell has slack and you make up the parts later in the same shift or week at no extra cost, you did not lose the margin — you just moved it. Be honest about which case you are in. Claiming lost margin on a line that runs 60% utilized is the fastest way to get your whole number thrown out.

Term 3: startup scrap

Restarting a process after an unplanned stop rarely produces good parts immediately. A molding press needs to come back up to temperature; a weld cell needs a few settling cycles; a coating line purges. Those rejected parts are a direct cost of the stop that a pure per-hour rate misses entirely.

Cost startup scrap at variable cost per part — the material and consumables you actually threw away — and count it per event, not per hour, because it depends on how often you restart, not how long you were down. Ten short stops create ten startups' worth of scrap; one long stop creates one.

Term 4: absorbed fixed overhead

This is the optional, contested term. Every scheduled hour carries a share of fixed overhead — depreciation on the machine, floor space, the baseline energy to keep the building conditioned, salaried supervision. When the machine is down, you are paying that overhead to produce nothing.

Accountants split on whether to include it. The case for: the overhead is real and the machine's job is to absorb it, so idle time is wasted absorption. The case against: you incur that overhead whether the machine runs or not, so it is not caused by the stop and does not belong in a marginal cost. Both views are legitimate. The clean answer is to report the number both ways — with and without overhead — and let the audience pick. Just never hide the choice.

A worked example: one month on a molding cell

Here is a month of unplanned downtime on a four-cavity injection molding cell running two eight-hour shifts, five days a week. Planned production time is 320 hours. Over the month the cell logged 40 hours of unplanned downtime across 50 separate stops — everything from material starvation to a barrel heater fault — with each stop recorded against a fixed reason-code list.

The cell's operating facts:

Convert each term to a cost per downtime hour:

So one hour of unplanned downtime on this cell costs about $800. Across the 40 hours logged that month, that is $32,000 — and if the month is representative, roughly $384,000 a year on a single cell.

Notice how the picture changes depending on which terms you keep:

All three are "the cost of downtime." They just answer different questions, and stating which one you mean is what makes the number defensible.

Two operators, base wage $22/hr, fully loaded at 1.36× → $30/hr each, so $60/hr of idle labor.
Running throughput of 300 good parts per hour.
Contribution margin of $2.00 per part (price minus variable material and energy).
The cell is the plant bottleneck — every lost hour is backorder — so lost margin applies in full.
Startup scrap of 10 parts per restart at $4.00 variable cost each.
Allocated fixed overhead of $90 per machine-hour.
Cash cost only (idle labor + scrap): $60 + $50 = $110/hr, or $4,400 for the month. This is money that physically left the building.
Opportunity cost (add lost margin): $710/hr, or $28,400. This is what the stops actually cost the business in unearned profit.
Fully loaded (add overhead): $800/hr, or $32,000.

Reading the result

The single most useful takeaway from the worked example is the ratio. Idle labor and scrap — the costs people instinctively reach for — are only $110 of the $800. The lost margin is $600, nearly three-quarters of the total. Plants that estimate downtime cost from labor alone routinely undercount by 5x or more, which is exactly why downtime projects get starved of investment: the business case is built on the smallest term.

The $800/hr figure also reframes improvement math. A $32,000-a-month problem justifies a real fix. If a $6,000 sensor and a preventive-maintenance change would eliminate the barrel-heater faults that caused eight of those 50 stops, the payback is measured in weeks, not years. You cannot make that argument without the cost number underneath it.

Where the number comes from

Every term in the formula depends on data you should already be capturing at the machine: how long each stop lasted, how many stops there were, and what caused each one. A cost calculation is only as good as the downtime log feeding it. Estimated durations, free-text reasons, and missed short stops all bleed straight into a soft cost number. If your log is not yet clean and reason-coded, fix that first — start with how to track machine downtime.

You also should not cost every stop equally. Spend your effort on the reason codes that dominate the total, which is what a downtime Pareto analysis surfaces. Cost the vital few, not the trivial many — putting a dollar figure on the top two or three codes is where the argument for a fix actually lives.

Common mistakes

Using selling price instead of contribution margin. Full price double-counts the material you never bought. Margin is the only defensible basis.
Claiming lost margin on a non-bottleneck. If you make the parts up later at no extra cost, the margin was deferred, not lost. Reserve this term for capacity-constrained lines.
Counting scrap per hour instead of per event. Startup scrap scales with the number of restarts. Ten short stops cost far more scrap than one long one of equal duration.
Hiding the overhead choice. Report the number with and without fixed overhead. Burying the assumption invites someone to "discover" it and discredit the whole figure.
Averaging across dissimilar machines. A bottleneck molding cell and a lightly loaded drill press have wildly different downtime costs. A single plant-wide per-hour rate is meaningless — cost each machine on its own facts.

Where HelpFab fits

The formula is arithmetic; the hard part is trustworthy inputs. In HelpFab's Issues and downtime tracking, operators log each stop against a machine from a phone or tablet using your own configurable issue types, and every event carries a start time, a stop time, and a reason code from a fixed pick-list. That gives you exactly the two numbers this calculation runs on — total minutes per code and the count of events per code — over any date range, without cleaning up free text first. Pull those into the four-term formula and the cost-per-hour number falls out.

Get the log right, cost the top reason codes honestly, and you turn "downtime is expensive" into a specific, defensible figure — the one that finally gets the fix funded.

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