Days' supply — the number that tells you not to buy

Sold volume alone will walk you into a price war. Days' supply compares how many are for sale against how fast they sell, and it is the difference between a busy segment and a crowded one.

Updated 1 August 2026 · 5 min read · Written for UK independent dealers

Most buying decisions are made on demand alone. That model sells well, so buy one. It is an intuition that works right up until it doesn't, and when it fails it fails expensively.

The missing half of the picture is supply. A segment where forty cars sold last quarter sounds healthy. If two hundred are currently advertised, it is not healthy — it is a queue, and you are proposing to join the back of it.

The calculation

Days' supply answers a single question: at the current rate of sale, how long would it take to clear the cars that are currently for sale?

Days' supply = active listings ÷ (sold in 90 days ÷ 90)

If 60 are advertised and 45 sold in the last 90 days, the daily rate is 0.5 cars a day, and 60 ÷ 0.5 = 120 days of supply.

That is the number. It is not how long your car will take to sell — it is a measure of how congested the market is, and therefore how much power you have over price.

Reading the number

The thresholds are not arbitrary. Below about 45 days, a buyer who wants that car has few alternatives and will pay for the one in front of them. Above about 150, any buyer can find eight of them within an hour's drive, and the only thing distinguishing yours is price.

Why this matters more for you than for a supermarket

In a glutted segment, the deciding factor is who can afford to be cheapest for longest.

A car supermarket with two thousand units, cheap capital and a national marketing spend can sit in a price war indefinitely. They can afford to hold a car at a thin margin because they are turning volume elsewhere and their cost of carrying stock is a fraction of yours.

You cannot. Your capital is finite and your forecourt has a fixed number of spaces. Every week you spend in a price war is a week your money isn't buying something that turns.

So a glut is not merely "a bit harder". It is a market you should structurally not enter, because the people you are competing with have advantages that no amount of good buying will overcome.

Volume can be a trap

The seductive error is a segment with high sold volume and high supply.

Take a very common hatchback. Three hundred sold in 90 days nationally — an enormous, liquid market. It feels like a safe buy precisely because it is so common.

Now look at the other side: nine hundred currently advertised. That is 270 days of supply. Every one of those nine hundred sellers is looking at the same strong sales figures and feeling reassured, and they are all about to compete with each other on price.

The high volume is real. It is also irrelevant, because the supply is higher still. Volume tells you the market exists; days' supply tells you whether there is room in it for you.

A big segment is not a safe segment. The safest segments are often small ones where hardly anything is for sale.

Whose sales are you counting?

One refinement worth making, and it changes the answer more than people expect.

Not all sales in a segment are available to you. If a model's sales are dominated by franchised dealers — because buyers of that car want the manufacturer warranty, or the finance offer, or the brand — then the headline sold figure overstates the demand an independent can actually capture.

The useful version of the calculation counts sales by independent dealers specifically. In some segments that is 70% of the market and the distinction barely matters. In others — nearly-new prestige cars, anything typically bought on manufacturer finance — it can be under a third, and the real market available to you is a fraction of the headline.

If you can get the split, use it. If you cannot, be sceptical of very strong demand figures in segments where you rarely see an independent selling one.

Supply moves, and it moves fast

Days' supply is a snapshot, not a constant. Fleet de-fleets, seasonal returns, a manufacturer discounting new cars into a segment, a lease cycle ending — any of these can double the supply in a segment within weeks.

Two practical habits:

Check before you buy, not from memory. A segment that was tight in March can be glutted by June. The figure you carry in your head is stale.

Watch the direction, not just the level. A segment at 100 days and falling is a better bet than one at 90 and rising. If you are checking regularly you will see the change, and the change is more informative than the level.

Using it in practice

Three places days' supply should change what you do:

Before bidding. Add it to the checks you run on any lot. Over 150 days, walk away regardless of how good the hammer price looks. The car is cheap because everyone else already worked this out.

When pricing your own stock. In a tight market you can sit slightly above the median and wait. In a heavy market you cannot — price keen from day one, because the third price cut in a glutted segment costs far more than pricing properly at the start.

When something is not selling. If a car is fairly priced, well presented, in season, and still sitting, check the supply. If the segment is glutted, that is your answer, and the response is to move the car decisively rather than to keep shaving the price while eight hundred competitors do the same.

The discipline

The thing that makes days' supply valuable is that it tells you when not to buy — and buying decisions are almost entirely framed the other way, as reasons to say yes.

A dealer who checks demand alone will buy into gluts several times a year, because glutted segments look busy. A dealer who checks both will pass on those cars and put the money into something scarcer, where they will hold their price and turn the unit in half the time.

Neither dealer can see the difference on the day. It shows up ninety days later, in the stock that is still sitting.