Product Mix Calculator: Throughput per Constraint Minute
When one resource limits the plant, the right question is not which product has the best margin but which earns the most per minute of the bottleneck. This calculator ranks two products by throughput per constraint minute and schedules the best mix inside the available time.
Your numbers
Raw material and purchased parts, plus any variable outside processing. Exclude labor and overhead.
Your results
Treats the constraint as the only limit and demand as firm. Check for other constraints, minimum order commitments and customer priorities before locking a mix.
Weekly throughput vs constraint minutes
Sweeps Constraint minutes available per week from 600 to 5000, holding your other inputs fixed.
Rank your whole product list by constraint minute
Optional: we can review how ERP routings and costs could rank every product by throughput per constraint minute.
Optional. The tool and every export work without this. No spam.
How to use it
Enter the minutes per week the constraint can actually run, then price, truly variable cost, constraint minutes per unit and weekly demand for each product. Truly variable cost is material and anything that rises one for one with units. Do not include direct labor or overhead, because they do not change with the mix this week. The calculator fills the constraint with the higher earning product up to its demand and gives the remainder to the other.
- Use constraint minutes net of downtime and setups
- Use firm demand, not forecast hopes
- Include only truly variable cost
- Re-run when the constraint or price changes
The method
Throughput per constraint minute = (price minus truly variable cost) / minutes on the constraint. With the defaults, A earns (140 - 60) / 8 = 10 per minute and B earns (95 - 35) / 3 = 20 per minute. B has the lower unit margin but earns double per bottleneck minute. B's 500 units use 1,500 minutes and A gets the remaining 900 minutes, which is 112 units, for a weekly throughput of about 30,000 + 9,000 = 39,000.
Reading the chart
The line rises steeply while extra constraint minutes go to the best product, then flattens in segments as demand is met. Where it becomes flat, more capacity at the constraint no longer adds throughput, and the constraint has moved or demand is the limit. That makes the chart a rough case for or against adding capacity at the bottleneck.
Common mistakes
Ranking by gross margin percent or by margin per unit is the standard trap and can reverse the right answer. Another is using full standard cost, which hides how much of it is fixed this week. Finally, making a plan for the constraint without protecting it from starvation wastes the benefit, because every lost constraint minute is lost throughput for the whole plant.
Frequently Asked Questions
What is throughput in this context?
Throughput is selling price minus truly variable cost, the money the system generates through sales. It is the term used in throughput accounting and the theory of constraints, and differs from gross margin because it excludes labor and overhead allocations.
Why not rank by margin percent?
Because the plant does not run out of percent, it runs out of constraint time. A high margin product that uses many bottleneck minutes can earn less per minute than a low margin one that uses few.
What if I have more than two products?
Rank them all by throughput per constraint minute and fill the available minutes from the top, limited by demand. This tool handles two; repeat it pairwise or use a worksheet to rank a longer list.
Should I always follow the ranking?
Use it as the starting point. Contract commitments, customer relationships and strategic items sometimes override pure throughput. The ranking shows the cost of those choices so they are deliberate.
We can pull price, bill of material cost and routing times from your ERP so this ranking is computed across your whole product list, not two products at a time.
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