Retail Math Formulas Every Boutique Owner Should Know

This page collects the retail formulas a small store actually uses, with one worked example each. Every dollar figure below is hypothetical, used to show the method rather than as a benchmark.

I'm Carina Hatton, boutique owner since 2013 and ecommerce coach since 2019. Where the site already has a calculator for a metric, I explain the formula here and send you there to run your numbers. The full set is on the free boutique calculators page.

Retail math cheat sheet

The short version. Each formula is explained further down.

Metric	Formula	What it tells you
Gross margin %	Gross profit ÷ selling price × 100	How much of the selling price you keep before operating costs
Markup %	Gross profit ÷ cost × 100	How far the price sits above what you paid
COGS	Beginning inventory + purchases + freight-in − purchase returns − ending inventory	The cost of the merchandise that actually sold
Inventory turnover	COGS ÷ average inventory at cost	How many times your inventory investment cycled
Sell-through rate	Units sold ÷ units received × 100	How much of a specific buy has cleared
GMROI	Gross margin dollars ÷ average inventory at cost	Margin dollars earned per dollar invested in stock
Open-to-buy	Planned sales + planned markdowns + planned ending inventory − beginning inventory − on order	How much buying room the plan still allows
Markdown %	Markdown dollars ÷ original retail × 100	How far a price has been reduced
AUR	Net sales ÷ units sold	The average price each unit actually sold for
IMU %	(Initial retail − cost) ÷ initial retail × 100	The margin built into the first ticketed price
Stock-to-sales ratio	Beginning-of-period inventory ÷ sales for the period (retail dollars)	How much stock you carry relative to what sells
Weeks of supply	Units on hand ÷ average weekly unit sales	How long current stock lasts at the recent pace
Sales per square foot	Net sales ÷ selling square feet	How productive your selling space is
ATV	Net sales ÷ number of transactions	Average dollars per customer transaction
UPT	Units sold ÷ number of transactions	Average number of items per transaction
Par level	A planned on-hand stock level you set and replenish back to	When a basic needs reordering

Margin and markup

Two percentages, the same gross profit dollars, different bases.

Example: an item costs $20 and sells for $50, so gross profit is $30. Markup is $30 ÷ $20 = 150%. Margin is $30 ÷ $50 = 60%. To convert between them, use decimals:

A 150% markup is 1.5, and 1.5 ÷ 2.5 = 0.6, so the margin is 60%. Going the other way, 0.6 ÷ 0.4 = 1.5. Margin has to stay below 100%, because a 100% margin would mean the product cost you nothing.

Confusing the two is an expensive mistake in a small store. The margin vs markup calculator does the conversion for you, including payment fees and per-sale costs.

Cost of goods sold

COGS is the cost of the merchandise that actually left the building during a period, not what you spent on orders.

Example: you start a quarter with $42,000 of inventory at cost, buy $56,000 more, pay $2,000 freight-in, return $1,000, and count $38,000 at the end. COGS is $42,000 + $56,000 + $2,000 − $1,000 − $38,000 = $61,000.

Revenue is what customers paid you. COGS is what that merchandise cost you. Keep COGS at cost rather than retail, because turnover, GMROI, gross margin and open-to-buy all take it as an input. Fees, ads, outbound shipping, rent and your own pay are operating expenses, not COGS. The COGS calculator walks the full inputs.

Inventory turnover

Example: COGS of $120,000 for the year, $42,000 of inventory at the start and $38,000 at the end. Average inventory is ($42,000 + $38,000) ÷ 2 = $40,000, so turnover is $120,000 ÷ $40,000 = 3.0.

You sold through the equivalent of your average stock investment three times. Turnover measures speed, not profit, and what counts as strong depends on category, price point, lead times and season, so compare your own periods and vendors rather than a number from elsewhere. The inventory turnover calculator also converts the ratio into days inventory held.

Sell-through rate

Example: you received 60 units of a style and 39 have sold in the window you are measuring. 39 ÷ 60 × 100 = 65% sell-through.

Sell-through measures how much of one specific buy has cleared over a period you define, which is a different question from how many units sold. Check it before you reorder or discount. The sell-through calculator runs it, and sell-through rate for boutiques covers the measurement window.

GMROI

Example: a category produced $48,000 in gross margin dollars on average inventory of $30,000 at cost. GMROI is $48,000 ÷ $30,000 = 1.60.

GMROI compares the gross margin dollars a category generated with the inventory investment required to generate them. A slow category with a fat margin and a fast one with a thin margin can look very different on turnover and very similar here. Compare categories and vendors against each other and against your own history rather than a published target. The GMROI calculator derives the margin dollars from net sales and COGS if your reports do not give them directly.

Open-to-buy

Everything in retail dollars, everything for the same period. Example: planned sales $20,000, planned markdowns $1,500, planned ending inventory $28,000, beginning inventory $26,000, already on order $8,000. Open-to-buy is $20,000 + $1,500 + $28,000 − $26,000 − $8,000 = $15,500.

It answers one question: can I order now, and for how much. It is not a bank balance and not a budget you pick, because it adjusts for stock on hand and stock already committed. A negative number means the plan is already covered. Use the open-to-buy calculator and the monthly open-to-buy method for small stores.

Markdown percentage

Example: a $60 dress marked to $42. Markdown dollars are $60 − $42 = $18, and the markdown percentage is $18 ÷ $60 × 100 = 30%.

Keep the four pieces separate. Original retail is the ticketed price, markdown dollars are the reduction, markdown percentage is that reduction against the original, and the new selling price is what the customer pays. What matters next is what the discounted price leaves after cost and fees, which is what the markdown calculator works out.

Average unit retail (AUR)

Example: a boutique sells 100 units and generates $4,800 in net merchandise sales. $4,800 ÷ 100 = $48 AUR.

AUR is the average price a unit actually sold for, after discounts, not the average price on your tags. It is useful in several ways:

• Change over time. A falling AUR with flat unit sales usually means discounting or a shift toward cheaper product.
• Category comparison. Accessories and outerwear will not share an AUR and should not be judged against each other.
• Markdown effect. AUR before and after a promotion shows what it cost you per unit.
• Positioning. An AUR well below the price band you think you occupy means customers are buying a different part of the assortment than you planned.

AUR is per unit and average transaction value is per transaction, so they only match when every customer buys one item. There is no correct AUR to aim for; it describes your assortment and your customer.

Initial markup (IMU)

Example: an item lands at $18 and is first ticketed at $45. IMU is ($45 − $18) ÷ $45 × 100 = 60%.

IMU is the margin built into the first price. Three related numbers get confused with each other:

• Initial markup is planned, measured on the first ticketed retail.
• Maintained margin is what survives after markdowns, promotions, employee discounts and shrink.
• Realized margin is the final figure your books show once everything has sold.

If that $45 item eventually sells at $36, the realized margin is ($36 − $18) ÷ $36 × 100 = 50%, not the 60% planned. That gap is why initial markup is set with expected markdowns in mind.

IMU is usually calculated on a retail base, which makes it look like gross margin, while everyday product markup is often quoted on a cost base. They are related but not interchangeable, so check which base a number uses before comparing it. The margin vs markup calculator moves between the two bases. There is no universal IMU target; the right one covers your expected markdowns and operating costs.

Stock-to-sales ratio

Both sides in retail dollars, both covering the same period. Example: you open the month with $30,000 of inventory at retail and sell $12,000 during the month. The ratio is $30,000 ÷ $12,000 = 2.5.

A higher ratio means more stock relative to what is selling, which can mean overbuying, slow demand or deliberate preparation for a peak. A lower ratio frees cash but raises the chance of running out of sizes and colors mid-month. Neither reading is automatically good.

Do not chase a published ideal. Category, seasonality, vendor lead time and business model all move the ratio, and a store that restocks weekly can run far leaner than one buying seasonally from overseas. Compare the same month across years and the same category across periods.

Weeks of supply

Example: 120 units on hand and 20 units sold in an average week. 120 ÷ 20 = 6 weeks of supply.

The answer comes out in weeks, which makes it easy to act on:

• Reorder timing. If weeks of supply is shorter than your vendor lead time, you will be out of stock before the replacement lands.
• Overstock. Styles with more weeks of supply than the season has left are markdown candidates, not reorder candidates.
• Seasonal planning. Counting back from an end date shows how much stock the remaining weeks can absorb.

It is a projection off recent pace, so recalculate it as demand shifts. More on using it week to week in the boutique inventory management guide.

Sales per square foot

Example: $46,000 in net sales over a quarter in 250 square feet of selling area. $46,000 ÷ 250 = $184.00 per square foot for the quarter.

Selling square feet means the space customers shop in, not stockroom or office, so online-only stores can skip it and use turnover and GMROI. Compare your own periods, or one area of the floor against another. The sales per square foot calculator also annualizes a short period.

Average transaction value (ATV)

Example: $12,000 in net sales across 300 transactions. $12,000 ÷ 300 = $40 ATV.

ATV is the basket number: it shows whether a promotion grew the basket or just discounted it, and whether upselling, bundles or spend thresholds are landing. It moves when item count changes, when prices change, or both. AUR is per unit and answers a narrower question.

Units per transaction (UPT)

Example: 450 units across 300 transactions. 450 ÷ 300 = 1.5 UPT.

UPT is the basket-building number, and what add-on merchandising, bundles, styling suggestions and register placement are trying to move. If a campaign was meant to get a second item into the bag, UPT is where you check.

The three are linked arithmetically: AUR × UPT = ATV. They do not move together automatically. A deep promotion can lift UPT while dropping AUR enough that ATV barely changes, and a shift to higher-priced product can lift AUR while UPT falls. Read all three before judging a promotion.

Par level

A par level is a planned on-hand stock level for a product you keep in stock continuously. When the count drops below par, you reorder back up to it. It is an operating rule rather than a single formula, and retailers set it several ways. A common approach for a replenishable basic covers the selling that happens while a reorder is in transit, plus a buffer:

Example: a basic tee in one size sells about 8 units a week, your vendor takes 3 weeks to deliver, you review the count weekly, and you want 6 units of cushion. Par is 8 × (3 + 1) + 6 = 38 units. That assumes steady weekly sales, a reliable lead time, and that you actually check the count on the review cycle. Other stores set par as a display minimum or a fixed shelf capacity instead, which is a legitimate operating choice.

Par levels suit basics and evergreen merchandise: the same item ordered again and again. They work badly for fashion and one-time buys, where sell-through, weeks of supply and markdown timing are the real questions.

Par level sets the stock level you order back up to. The trigger that tells you the moment has arrived is the reorder point: average daily unit sales × lead time in days + safety stock. The reorder point calculator works it out per product and shows the arithmetic.

How the numbers relate to each other

Each formula answers a narrow question, and they are most useful read together. AUR, UPT and ATV describe the sale itself, and ATV equals AUR × UPT. Sell-through and weeks of supply describe a specific buy and how long it lasts. Stock-to-sales, turnover and GMROI describe the whole inventory investment and what it earns. Open-to-buy converts all of it into how much you can still spend.

These describe the same business from different angles, so a change in one often shows up in another. That is not the same as one causing the other. Rising turnover could come from better selling or from having bought too little, and the formula cannot tell you which. The numbers narrow the question; the floor, the vendor and the season answer it.

Which retail metric should I use?

Question	Metric
Are products selling through?	Sell-through rate
How quickly is my inventory investment cycling?	Inventory turnover
How much gross margin am I earning from inventory investment?	GMROI
How much can I still spend on inventory?	Open-to-buy
What is my average selling price per unit?	AUR
How much does an average customer transaction spend?	ATV
How many items are in an average purchase?	UPT
How long will current stock last?	Weeks of supply
How productive is my physical selling space?	Sales per square foot
Is my price high enough to survive markdowns?	IMU and gross margin
Can I afford this discount?	Markdown percentage and margin after fees

Using these without a benchmark

Plenty of pages offer an ideal turnover, an ideal sell-through or a good margin percentage. Most of those figures come from a part of retail that looks nothing like an independent boutique, and applying them can push you into buying or discounting decisions that do not fit your store.

Compare against things you actually have: this month against the same month last year, one category or vendor against another, one season against the last, and your own historical performance. Pick three or four numbers to run monthly rather than all sixteen. For most small stores that is sell-through on recent buys, weeks of supply on the styles you care about, turnover or GMROI by category, and open-to-buy before the next order. If you need the commercial context before the formulas, begin with how wholesale differs from retail. Pricing decisions are covered in the retail pricing guide.