# Scale _Roughly right beats precisely wrong_ --- How many petrol stations are there in the UK? Let's work it out. UK population: roughly 70m. How many cars? Not every person, but most households have one. Assume 1 for every two people to keep the maths simple - 35m. How often does a car fill up? Weekly feels too often, monthly too seldom. Twice a month - so 15m-20m cars filling up every week, or 2m-3m per day. How many cars can a station serve per day? 1 per hour feels low, 100 per hour too high - let's go with 10 per hour, or 200-300 per day. Picking the easier numbers: 3m cars filling up per day, with each petrol station filling up 300, equates to 3,000,000 / 300 = 10,000 petrol stations in the UK. The real answer is about 8,300. We were off by 20%, using five estimates, each of which could have been wrong by half. It doesn't matter - we were never going to land on 800 or 80,000. --- The same thinking on a business question. Practice management software for UK dental surgeries. At least one practice on most high streets - call it 10,000-15,000 nationally. Each paying a few hundred to low thousands per year. The market is tens of millions. You don't know if it's £25m or £75m - but it's not £5m or £500m. That's usually enough to size your bet. --- Once you can do this, you start catching things in ordinary meetings. Your sales leader proposes hiring five new reps. The business case says they'll each close £500k in their first year. That's £2.5m of new revenue. The plan looks reasonable on the slide. But you know your average deal size is about £5k. So £500k per rep means each one closing a hundred deals in their first year. That's two a week from a standing start. And five reps doing that means five hundred new customers - you currently have two hundred. The plan is projecting a 250% increase in the customer base in twelve months. You didn't check the spreadsheet. You multiplied three numbers in your head. The plan isn't ambitious, it's impossible. Most serious errors aren't 10% off. They're 10x off. They're obvious if you have a feel for the right neighbourhood. Round numbers and rough maths - don't underestimate it. ---