Why do tail sizes come out at one or two units per door?

Because a percentage applied to a small number produces a small number, and then rounding does the rest. A size taking 5% of a 600-unit buy across 30 doors is one unit per door before any rounding at all. The size curve is not wrong — it is being asked to distribute a quantity too small for the run it describes, and the smallest percentages hit that limit first.

How do you calculate the minimum buy depth for a size run?

Work backwards from the floor. Multiply the minimum presentable quantity per size per door by the number of doors, then divide by the smallest percentage in the size curve. The result is the buy quantity below which the run breaks on arrival. Anything under that figure means at least one size cannot be presented properly anywhere, no matter how the allocation is arranged.

What is minimum presentable quantity?

The smallest number of units of a size a door needs to display it as a real offer rather than a remnant — a floor-set and merchandising number, set by fixture and category convention. It is not the same as a replenishment trigger, which is the stock level at which a reorder fires. The two get conflated because both are small numbers per size per door, but one governs whether the size can be shown at all and the other governs when it is topped up.

What can you do if the buy cannot be made deeper?

There are four levers: increase the buy, shorten the size run, reduce the number of doors carrying the full run, or profile the allocation by door cluster. The fourth is the one an experienced allocator reaches for first — send the full run only to doors whose cluster skews toward the tail sizes, and a truncated run elsewhere. That preserves both breadth and door count, and it turns the depth floor into a per-cluster number rather than a chain-wide one.

How can you tell a buy was too shallow after the season?

Look for a by-size sell-through curve that is high at both ends and low in the middle. That shape is the signature of integer rounding: the tail sizes were shipped in ones and twos, sold out immediately and read as strong, while the units rounded out of them were pushed into the core sizes, which finished over-stocked. It is easy to misread as a size-curve error and re-weight toward the tail, which makes the following season worse.

// Guide

How to find the minimum buy depth a size run can support

A size curve tells you how to split a buy. It does not tell you whether the buy is large enough for the split to mean anything. Below a certain depth, the smallest percentages in the curve resolve to one unit per door — and a size shipped in ones is not an offer.

The floor is calculable, and it is usually higher than people expect.

Use the Size Curve Calculator →

Quick answer

Minimum buy = (minimum presentable quantity per size per door × number of doors) ÷ the smallest percentage in the size curve. Below that quantity, at least one size cannot be presented properly in any door, regardless of how the allocation is arranged.

Definition — Minimum presentable quantity: Minimum presentable quantity is the smallest number of units of a size a door needs to show it as a genuine offer rather than a remnant. It is a floor-set and merchandising number set by fixture and category convention, and it is distinct from a replenishment trigger, which governs when stock is topped up rather than whether it can be displayed at all.; minimum buy = (MPQ × doors) ÷ smallest curve percentage; Used by: Buyers and allocators checking whether a planned buy can carry its size run; Related: Size curve, buy plan, door clustering, allocation

// The arithmetic

Why the tail collapses first

Take a 600-unit buy across 30 doors, with a size curve whose smallest position takes 5%.

600 × 5% = 30 units for that size, chain-wide
30 units ÷ 30 doors = 1 unit per door

No rounding error is required to get there. The curve is correct, the allocation is correct, and the size still arrives as a single unit in every door — where it either sells on day one and is absent for the rest of the season, or sits as the only one of its kind on the fixture. The smallest percentages hit this limit first, which is why the problem always presents at the ends of the run.

// The floor

Calculating the depth floor

Reverse the calculation. If a size needs at least a given quantity per door to be presentable:

minimum buy = (minimum presentable quantity × doors) ÷ smallest curve percentage

Continuing the illustrative example above, with a minimum presentable quantity of 2:

(2 × 30) ÷ 0.05 = 1,200 units

So the 600-unit buy is half what this size run needs. That is the useful output: not “the curve is wrong” but “this run needs 1,200 units or it needs to change shape.”

// Four levers

What to do when the floor is not met

1. Buy deeper. Correct when the style justifies it, and the option most often unavailable.
2. Shorten the size run. Drop the smallest positions. This raises the smallest remaining percentage, which lowers the floor — the denominator in the formula is doing the work.
3. Reduce door count. Fewer doors carrying the style, each carrying it properly. Trades reach for presentation.
4. Profile the allocation by door cluster. The one an experienced allocator reaches for first. You do not have to send every size to every door: send the full run to clusters that skew toward the tail sizes and a truncated run elsewhere. This preserves breadth and door count simultaneously, and it makes the floor a per-cluster number rather than a chain-wide one — which is usually a much easier number to meet.

// Diagnosis

The tell in the hindsight data

A by-size sell-through curve that is high at both ends and low in the middle is the signature of a buy that was too shallow for its run. The tail sizes shipped in ones and twos, cleared immediately and look like the strongest sizes in the range. The units that rounding pushed out of them landed in the core sizes, which finished over-stocked and look weak. The natural response — re-weight the curve toward the tail next season — makes it worse, because the curve was never the problem. Check the depth floor before re-cutting the curve.

// FAQ

Frequently asked questions

Why do tail sizes come out at one or two units per door?: Because a percentage applied to a small number produces a small number, and then rounding does the rest. A size taking 5% of a 600-unit buy across 30 doors is one unit per door before any rounding at all. The size curve is not wrong — it is being asked to distribute a quantity too small for the run it describes, and the smallest percentages hit that limit first.
How do you calculate the minimum buy depth for a size run?: Work backwards from the floor. Multiply the minimum presentable quantity per size per door by the number of doors, then divide by the smallest percentage in the size curve. The result is the buy quantity below which the run breaks on arrival. Anything under that figure means at least one size cannot be presented properly anywhere, no matter how the allocation is arranged.
What is minimum presentable quantity?: The smallest number of units of a size a door needs to display it as a real offer rather than a remnant — a floor-set and merchandising number, set by fixture and category convention. It is not the same as a replenishment trigger, which is the stock level at which a reorder fires. The two get conflated because both are small numbers per size per door, but one governs whether the size can be shown at all and the other governs when it is topped up.
What can you do if the buy cannot be made deeper?: There are four levers: increase the buy, shorten the size run, reduce the number of doors carrying the full run, or profile the allocation by door cluster. The fourth is the one an experienced allocator reaches for first — send the full run only to doors whose cluster skews toward the tail sizes, and a truncated run elsewhere. That preserves both breadth and door count, and it turns the depth floor into a per-cluster number rather than a chain-wide one.
How can you tell a buy was too shallow after the season?: Look for a by-size sell-through curve that is high at both ends and low in the middle. That shape is the signature of integer rounding: the tail sizes were shipped in ones and twos, sold out immediately and read as strong, while the units rounded out of them were pushed into the core sizes, which finished over-stocked. It is easy to misread as a size-curve error and re-weight toward the tail, which makes the following season worse.

// Summary

A correct size curve still fails if the buy is too small: the smallest percentages resolve to one unit per door.
Minimum buy = (minimum presentable quantity × doors) ÷ smallest curve percentage.
Minimum presentable quantity is a display floor, not a replenishment trigger — they are different numbers.
Four levers: buy deeper, shorten the run, cut door count, or profile the allocation by door cluster.
A sell-through curve high at both ends and low in the middle means depth, not curve shape, was the problem.

// Related resources

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// Next step

Your calculator result is one number. RetailNorthstar keeps the whole plan connected — line plan, OTB, assortment, buy, POs, and production.

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