- Maths reviewed by Josh Miller
- Tested by Diana Westfield
The betting variance calculator runs your bets hundreds of times so you can see what luck looks like. £10 bets at 2.10 with a genuine 3% edge, 500 of them. You expect +£150. But in about one run in four you’re still behind after all 500, and a dip of £200 or more along the way is normal. Change the numbers and press “Run again”. It’s free and it does a fresh simulation every time.
Betting variance in plain numbers
At 2.10 with a 3% edge, you win 49% of bets. Each bet’s result swings by about £10.50 either side of its 30p average. Over 500 bets that swing grows with the square root, to about £235, while the average only grows to £150. So one standard deviation is bigger than the profit you’re aiming for.
That’s the whole problem. A real edge is small, and it hides behind the noise for a long time.
Reading the chart
Each coloured line is one possible run of your bets. The dashed line is the average. Some lines finish well above it, some below zero, and almost all of them wander. If your last two months look like one of the low lines, that’s not proof your method’s broken. It’s also not proof it works. The luck vs skill calculator helps with that question.
Odds change everything
Keep the edge at 3% and move the odds. At 1.50, swings shrink and you’re down after 500 bets about 17% of the time. At 5.00 it’s nearer 37%, and the drawdowns are much deeper. Longshot bettors need bigger banks and more patience, even when they’re right.
| Average odds (3% edge) | Chance down after 500 bets | Chance down after 2,000 |
|---|---|---|
| 1.50 | about 17% | about 3% |
| 2.10 | about 26% | about 10% |
| 3.00 | about 32% | about 17% |
| 5.00 | about 37% | about 25% |
Sizing for the swings
Add a starting bankroll and the CasinoCalcs calculator counts the runs that go bust. With £500 and £10 bets (50 stakes) at 2.10, fewer than 1 run in 100 goes bust. Halve the bankroll and it’s about 1 in 7, with exactly the same edge. The betting bankroll calculator turns that into a unit size, and the Kelly criterion calculator sizes bets from the edge itself.
Set the edge to 0, or to −5% for a typical bookmaker margin, to see what betting without an edge looks like over time. It’s a sobering chart.
To watch those swings play out, the betting strategy simulator runs thousands of seasons and compares staking plans.
Losing runs are normal. Chasing them is where the damage happens. BeGambleAware.org is free if you’re finding it hard to stop.