Mark-Up vs Margin: The Sum Heating Engineers Get Wrong

Mark-up and margin: the difference in plain English

Same job, same cost. The number you keep depends entirely on which sum you did.

Here is the thing. Mark-up and margin sound like two words for the same idea. They are not. They are two different sums that spit out two different numbers, and the whole trade uses them as if they are interchangeable.

Mark-up is what you add on top of your cost. If a job costs you 100 pounds and you add 25 pounds, that is a 25 percent mark-up. Simple. You multiply your cost by a percentage and stick it on the end.

Margin is different. Margin is the share of the final invoice that is profit. On that same job you charged 125 pounds and kept 25 pounds, so your margin is 25 divided by 125, which is 20 percent. Same 25 quid. Two different percentages. One is measured against your cost, the other against your price.

That is the whole trick. Mark-up looks backwards at what you paid. Margin looks at what the customer paid. And because your price is always bigger than your cost, your margin is always a smaller percentage than your mark-up. Every single time.

Why does this matter to a heating engineer more than most? Because your jobs carry heavy materials. A boiler, a cylinder, rads, controls, sundries. When a big chunk of the invoice is stuff you bought in, the mark-up versus margin gap gets wider, and the money you think you are making drifts further from the money in the bank. If you have ever had a cracking year on paper and an empty feeling in the account, this is often where it starts.

Why nearly every heating engineer gets it wrong

Let us cut to the chase. Nobody sat you down at college and taught you this. You learned to price the way the engineer who trained you priced, and he learned it off the one before him. The rule of thumb got passed down like a dodgy hand tool, and nobody checked whether it actually worked.

The classic version goes like this. "I want to make 30 percent, so I add 30 percent." Sounds right. Feels right. It is wrong. Adding 30 percent is a mark-up. It leaves you a 23.1 percent margin. You just gave yourself a pay cut and did not notice.

I have spent years looking at plumbing and heating accounts, and this is the single most common leak I see. Not overspending. Not slow customers. A pricing sum that was quietly wrong on every job for years. The business looked busy, the owner looked knackered, and the profit was thin as a shim washer.

The rule of thumb gets passed down the trade. Nobody checks whether it survives contact with the maths.

Let us face it, the reason it survives is that it is invisible. A wrong mark-up does not bounce a payment or throw an error. The job still gets done, the customer still pays, the money still lands. It just lands a bit light, every time, and a bit light times two hundred jobs a year is a holiday, a van, or a pension you never took.

The other reason is confidence. Percentages feel like maths, and maths feels certain. So once you have a number you trust, you stop questioning it. That is exactly the mindset that keeps the mistake alive. This is a classic case of working IN your business instead of ON it. The one afternoon you spend fixing the sum pays back more than a fortnight on the tools.

If you want the bigger picture on where profit leaks out of a plumbing and heating business, I broke down all of it in the nine drivers of profit. Pricing is only the first one. But it is the one that costs the most and takes the least time to fix.

The 1,286 pound mistake: a worked example

Numbers make this real, so let us do one properly. Take a full system job. Nothing exotic. Your true cost, once you have added up the boiler, the cylinder, the rads, the controls, the sundries and your labour, comes to 10,000 pounds.

You want to make 30 percent. So you add 30 percent. Your price is 13,000 pounds. Your profit is 3,000 pounds. You feel good. You made your 30 percent, right?

Wrong. Your margin on that job is 3,000 divided by 13,000, which is 23.1 percent. You wanted 30 and you got 23.1. Sound familiar?

Now scale it. This is not a one-off. It is your default. Every job priced the same way carries the same silent shortfall. Do a couple of full jobs a month plus the service and repair work, and you are not talking about a lost holiday. You are talking about the difference between a business that pays you properly and one that keeps you awake at 3am.

And here is the part that stings. You did not have to sell harder, work later, or squeeze a single supplier. The customer was happy to pay a fair price. You just asked for the wrong one. That is the good news hiding inside the bad news: this is the cheapest profit you will ever recover, because it costs you nothing but a corrected sum.

The mark-up to margin conversion table

Pin this to the van wall. This is every common mark-up, what margin it actually leaves you, and the profit it produces on a job that costs you 10,000 pounds. Read across and watch the gap between what you think you are making and what you are really making.

Notice the shape of it. A 20 percent mark-up leaves 16.7 percent margin. A 50 percent mark-up leaves 33.3 percent. You have to add double your cost, a 100 percent mark-up, just to keep half the invoice. The bigger the number, the wider the gap. That is why the "just add 20 percent" habit hurts most on your biggest, most material-heavy jobs.

Now flip it round, because this is the version you actually price with. If you know the margin you want to keep, this is the mark-up you must add to get there.

If you would rather never do this in your head again, the maths lives inside a proper quoting process. I built a free job estimate spreadsheet that does the margin sum for you, so labour, materials and profit come out right without a calculator.

Work it out on your own job

Enough theory. Put your own numbers in. Type in what a typical job costs you, the margin you actually want to keep, and the mark-up you have been adding out of habit. The calculator shows you the right price, the margin your old habit was really giving you, and the gap in pounds.

Most engineers put their real numbers in and go quiet for a second. That gap is not a rounding error. It is the deposit on a decent van, sat in the space between two sums. And it is there on every job, whether you look at it or not.

The margin-first method: divide, do not multiply

One rule replaces the guesswork: price equals cost divided by one minus your margin.

Right, the fix. It is one rule, and it is the most valuable line in this whole article, so read it twice.

To price for a margin, divide your cost by one minus your margin. Do not multiply by one plus a percentage. Multiplying gives you a mark-up. Dividing gives you a margin. That is the entire difference.

That is it. No spreadsheet gymnastics, no accountant on speed dial. One sum, done the right way round, on every quote. The reason it works is that it starts from where your money actually is, the invoice, and works back. Mark-up starts from your cost and hopes for the best.

I call a spade a spade, so here is the blunt version. If you take nothing else from this, take the divide sign. Swap your times sign for a divide sign on every quote, and you have given yourself a pay rise the customer will never even see on the price.

Once your per-job margin is right, the next lever is your whole pricing model. Whether you charge project prices, day rates or per unit changes how the margin behaves across a year, and I compared all three in project versus day rate versus per square metre pricing.

Materials: where the leak is worst

Material-heavy jobs widen the mark-up versus margin gap. Prices are climbing too.

Heating work lives or dies on materials, and materials are exactly where the mark-up versus margin gap bites hardest. The more of your invoice is bought-in kit, the more a wrong sum costs you.

It is worse than usual right now. UK material prices rose 4.7 percent across February to April 2026, according to the Builders Merchant Building Index, with renewables and water products up a punchy 17.5 percent over the year. If you are still adding the same mark-up you used in 2023, inflation has been quietly eating your margin from the other side while the wrong sum eats it from this side.

The trade argues about material mark-up constantly, and the honest ones admit the number needs to cover more than the sticker price. On the Screwfix Community Forum, one member put the going rate at 25 percent while noting plenty charge far more to cover the time and hassle. Another explained he charges retail so he keeps his trade discount, landing at 15 to 30 percent mark-up. They are not being greedy. They are recovering the real cost of sourcing, collecting, storing, carrying and guaranteeing the gear.

So separate the two jobs in your head. First, mark materials up enough to cover the genuine cost of handling and risk. Second, set your overall margin on the whole invoice using the divide rule. One protects you on parts. The other protects you on the business. Presentation matters too, and how you lay materials out on the page changes how customers react to the price, which is the whole point of the 12-section boiler quote structure.

Your repricing checklist

Do not overhaul everything at once. Work through this in order. An afternoon with a brew and your last three months of jobs is all it takes.

1. Pull your real costs: for a typical job, add up materials at what YOU pay, labour at your true charge-out cost, plus a slice of overhead, van, fuel and wastage. Guessing here undoes everything downstream.
2. Check your last three quotes: divide the profit by the price on each. That is your real margin. Write it down and sit with how it compares to what you thought.
3. Pick a target margin: decide the share of every invoice you intend to keep. For most heating work, somewhere between 25 and 40 percent is the honest range once overheads are in.
4. Switch to the divide rule: price every job as cost divided by one minus your margin. Bin the "add a percentage" habit for good.
5. Set a separate materials mark-up: cover sourcing, storage, transport and warranty risk before profit. Do not let parts leave at cost.
6. Rebuild your rates for 2026 prices: material inflation means last year's numbers are already out of date. Re-cost your common jobs against today's merchant prices.
7. Watch the win rate, not your nerves: put the new prices out and track how many jobs you actually win. If you are still winning most quotes, you were too cheap, not too dear.
8. Review it quarterly: costs move, so your prices should too. Diarise one afternoon every three months to run the sums again.

Rising costs with customers who will not pay more is its own puzzle, and I worked through the practical repricing moves in detail in how to reprice your P&H business when costs rise. This checklist is the pricing engine. That article is the wider strategy around it.

What the trade is saying

This is not just accountant talk. The confusion, and the honest reckoning with what a fair number really is, plays out in the trade forums every week. Here is a sample of real voices.

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Frequently asked questions

My verdict

For more plain-English money guidance built specifically for plumbing and heating businesses, my team and I publish at Together We Count. Get the pricing right first. Everything else in the business gets easier from there.