Stockout cost is the profit you lose when a customer wants a product you don't have, plus what you spend to fix the gap. Count the margin on sales that really walk out, not the price. In a worldwide study, 40% of shoppers who met a gap bought elsewhere or not at all.
The quick formula most guides give, days out × daily sales × price, measures sales at risk and overstates the loss several times over. This guide builds the real number for one Dar es Salaam shop in Tanzanian shillings, then shows which products deserve a bigger buffer.
What Is Stockout Cost?
Stockout cost is the total a business loses because a product was not there when a customer wanted it, a stockout. It has three parts: margin lost on sales that walked out, money spent to end the gap early, and later margin from customers who left.
Stockout cost = (lost units × margin per unit) + rush costs + margin lost on customers who leave
Lost units are the sales that went to a competitor or nowhere. Sales that switched to another product you sell, or came back later, mostly stayed with you.
How to Calculate Stockout Cost
The formula most guides give is stockout cost = days out of stock × average units sold per day × price per unit. It measures sales at risk: the most a gap could cost, not what it did cost.
Calculate the real cost in four steps. Every input comes from your sales records, your product costs and a few weeks of notes at the till.
- Units at risk = days out of stock × average units sold per day.
- Units lost = units at risk × the share of customers who walk out. Use 40% until your own notes say otherwise.
- Margin lost = units lost × (price − cost), plus the margin you gave up on units sold as a cheaper substitute.
- Rush costs = the extra you paid per unit, plus the transport, for any emergency purchase.
The 40% comes from a worldwide study of grocery shoppers by Gruen, Corsten and Bharadwaj: 31% bought at another store and 9% did not buy. Another 26% switched brand, 19% took another size or variety, and 15% bought later (summarised in a 2017 journal paper).
For example: a Dar es Salaam shop out of cooking oil for 4 days
For example, a mini supermarket in Dar es Salaam sells 20 bottles of 1-litre cooking oil a day. It buys at TZS 4,700 and sells at TZS 5,500, a margin of TZS 800 a bottle. The distributor's truck is late, and the oil is out for 4 days.
On day 4 the owner sends a bajaji to a Kariakoo wholesaler for 2 cartons of 12. They cost TZS 300 more per bottle, plus TZS 15,000 for the ride.
The shop stocks a second oil brand that earns TZS 600 a bottle, TZS 200 less. The customers who met the gap split the way the study found.
| Layer | How it is counted | TZS |
|---|---|---|
| Sales at risk (the quick formula) | 4 days × 20 bottles × 5,500 | 440,000 |
| Walked out: 40% of 80 bottles | 32 bottles × 800 margin | 25,600 |
| Took the other brand or size: 45% | 36 bottles × 200 less margin | 7,200 |
| Came back after the delivery: 15% | 12 bottles, sold later | 0 |
| Rush purchase from Kariakoo | 24 bottles × 300 + 15,000 transport | 22,200 |
| Stockout cost you can count | 25,600 + 7,200 + 22,200 | 55,000 |
The quick formula says TZS 440,000. The money actually lost is TZS 55,000, one eighth of that, and about 40% of it is the rush purchase.
A shop with no second brand would lose most of the 36 substitute sales as well. At 68 bottles × 800, the margin lost rises from TZS 32,800 to TZS 54,400.
Go deeper: the regular customer who doesn't come back
The largest cost often comes later and shows in no report. Suppose one regular who spends TZS 30,000 a week, at a 15% margin, shops next door for six months instead.
That is 30,000 × 15% × 26 weeks = TZS 117,000, more than twice the visible cost. Count it only when you know the customer left; the hidden costs below explain the rest.
What Makes Up Stockout Cost?
Stockout cost is made of five losses, and only the first shows in the day's figures. The rest need a note, an estimate or a follow-up.
Lost sales
Lost sales are the units customers wanted and then bought elsewhere or not at all. Count them at margin, not price, because you also never paid for the goods you didn't sell.
Lost customer lifetime value
A regular who meets the same gap twice may move their whole basket, not just that item. Their lifetime value is weekly spend × your margin × the weeks they would have kept coming.
Cancelled orders and refunds
A customer who paid a deposit or ordered by phone for something you can't supply gets the money back. You lose the margin, the time spent on the order, and often the customer's next order too.
Extra customer support
Every "when will it come?" call or WhatsApp message takes a staff member away from selling. At one counter it is minutes; with online orders already paid, it can be hours a day.
Emergency restocking costs
A rush order means a higher unit price at the nearest wholesaler, a boda boda or bajaji fare, and often a smaller pack. In the example it was TZS 22,200, about 40% of the visible cost.
What Drives Stockout Costs Higher?
Five things raise the cost of a gap. They are how long it lasts, what each sale earns, who the customer is, what else you offer, and when it happens.
Long lead times
A gap lasts until the next delivery arrives. A product imported through Dar es Salaam or Mombasa can be weeks away, so a missed reorder costs weeks of margin, not days.
High-margin products
The cost per lost unit is the margin. A phone charger that earns TZS 6,000 costs more per missed sale than seven bottles of oil, so high-margin lines deserve their own reorder point.
Loyal or repeat customers
Gaps on the items your regulars buy every week put their whole basket at risk. The TZS 117,000 regular above is the cost of getting this wrong once.
Limited alternatives
When the shop has no other brand or size, the 45% who would have switched have nothing to switch to. In the example, that raises the margin lost by two thirds.
Time-sensitive demand
A gap in the week school opens, before Eid or before Christmas is not made up later. The 15% who would have waited buy elsewhere instead, because their need has a date.
Stockout Cost vs. Holding Cost
Stockout cost is what you lose by holding too little; holding cost is what you pay for holding too much. The right buffer is where an extra unit costs about as much to hold as the stockouts it prevents.
Stockout cost
Stockout cost arrives in lumps: nothing for weeks, then TZS 55,000 in four days. It falls hardest on fast sellers whose customers walk out.
Holding cost
Holding cost is paid every day on every unit: the money tied up, the space, damage, theft and expiry. The holding cost guide worked out 39.8% a year for one Kampala shop; work out your own.
Why balance matters
For example, riding out a 4-day late delivery of the oil needs 80 extra bottles. At TZS 4,700 each that is TZS 376,000 of stock, which costs TZS 150,400 a year to hold at 40%.
That buffer pays for itself if it prevents three such gaps a year, because 3 × 55,000 = TZS 165,000. It pays at one gap a year if that gap would also have cost a regular customer.
| Product | Stockout cost | Holding cost | What to do |
|---|---|---|---|
| Fast seller, customers walk out (cooking oil, sugar, flour) | High | Low | Keep a full buffer |
| Slow, expensive item customers will order (fridges, phones) | Low | High | Small buffer; take a deposit and order |
| Short shelf life (milk, bread, fresh produce) | Medium | High: what doesn't sell spoils | Small buffer, more frequent deliveries |
| Seasonal (uniforms, exercise books before term) | Very high in the season | Low before, high after | Stock up early, let it run out after |
How to Reduce Stockout Cost
Reduce the cost by making gaps rarer, shorter and cheaper. Six habits do it, and the linked guides work each one through.
Keep safety stock
Size the buffer by the table above, not by one rule for everything. How to avoid overstocking and stockouts compares three ways to size it.
Set reorder points
A reorder point tells you to order before the shelf empties: average daily sales × lead time + safety stock. Enter it as each product's Stock Alert so ArmPOS warns you.
Forecast demand
Raise stock before school terms, festive weeks and paydays, using last year's sales for the same weeks. See inventory forecasting methods.
Improve supplier reliability
Log every order's date and delivery date, and keep a second supplier for your top sellers. A late supplier costs you the gap, the rush purchase and possibly the regular.
Watch your cash flow
A shop that can't pay on the day misses the order the alert asked for. Keep cash for the best sellers first, before slow lines and before more credit to customers.
Use inventory software
Software that warns you when stock reaches the reorder point catches a gap before customers do. ArmPOS checks stock every 10 minutes and alerts the people at that warehouse; you still place the order.
The Hidden Impact of Stockout Cost
The hidden costs arrive after the shelf is full again, which is why most shops never count them. Five are worth knowing, even when you can't put a number on each.
Missed long-term revenue
Customers who found stock at another shop may keep buying there. The TZS 117,000 regular in the example is this cost.
Lower customer trust
Shoppers remember which shop always has the basics. Corsten and Gruen titled their summary of the worldwide study stock-outs cause walkouts, and repeated gaps turn one walkout into a habit.
Bad reviews
A review that says "never has stock" reaches shoppers who have never visited. An online order you can't fill is the quickest way to earn one: the customer paid, then waited.
Higher customer acquisition costs
Every regular you lose has to be replaced with promotions, flyers or discounts. That spend belongs to the stockout, even though it lands months later in the marketing budget.
Drop in conversion rates
An out-of-stock product page in an online shop gets visits and no sales, so its conversion rate falls. ArmPOS's online shop uses the same stock as the counter, so a gap at the till is a gap online too.
How to calculate stockout cost with ArmPOS
ArmPOS holds four of the five inputs; the walk-outs need a notebook. Here is where each number comes from.
- Average daily sales. Open Reports › Products report for the 30 days before the gap, and divide the product's quantity by the days it had stock (Stock reports).
- Margin per unit. In Products › All Products, click View on the product. Its page shows Cost and Price, and the margin is the difference (Find and edit products).
- Days out of stock. Notifications › Inbox keeps every alert that reached the bell, by day. It shows when the product ran low and when it ran out (Get alerts on your phone). The delivery that ended the gap is in Purchases › All Purchases.
- Walk-outs. No till counts customers who leave without buying. Keep a notebook at the counter for two weeks, noting each request you couldn't fill and whether the customer took something else.
- Rush costs. Record the emergency purchase as it happened. Enter the higher cost on the line with its pencil, and the transport in Shipping (Record a purchase).
Then set the product's Stock Alert to its reorder point. Turn on the "Stock fell below its alert level" alert, so the next gap is flagged days ahead. The steps are the same for a shop in Tanzania and for each branch on its own.
Frequently asked questions
Is stockout cost the same as lost sales?
No. Lost sales are the revenue that walked out; stockout cost is the margin on it, plus rush costs and customers who don't return. In the example, TZS 440,000 of sales at risk became TZS 55,000 of cost.
Should I use the selling price or the margin?
Use the margin. You never paid for the goods you didn't sell, so only the profit on them is lost.
How do I know how many customers walked out?
Keep a notebook at the till for two weeks. Until then, use the worldwide study's 40% (31% bought elsewhere, 9% didn't buy) as a starting estimate.
Is a stockout worse than overstocking?
For a fast seller, often yes. In the example, one gap that also loses a regular costs TZS 172,000, more than a year of holding the buffer (TZS 150,400). For slow, expensive or perishable stock, overstocking costs more, so the buffer differs by product.
Pick the five products whose gaps hurt most and work out one stockout cost for each with the table above. Create a free ArmPOS account to set their reorder points as Stock Alerts, so you hear about the next gap before a customer does.
