The Odds Lab

How to Trade on Betfair: A Beginner's Guide

Most people arrive at an exchange from traditional betting. They know what odds are, they know what a stake is, and they know that a bet is settled when the match finishes. Trading works differently. The position can be opened and closed before a ball is kicked, and the result of the event may end up being irrelevant to the outcome of the trade.

This piece covers the entry point: what back and lay actually mean, why a position can be closed early, the arithmetic of a green-up worked through in full, and what commission, liquidity and the spread do to the numbers. It also says plainly that the mechanics are the easy part, and that most people who try this lose money. Anyone already comfortable with back and lay may prefer the wider survey of pre-match trading approaches, or a single strategy examined in detail in Lay the Draw.

An exchange is a marketplace, not a bookmaker

A bookmaker sets prices and takes the other side of every bet. An exchange does neither. It matches one customer against another and charges a commission on net winnings.

That structural difference is what makes trading possible. On an exchange, prices move because participants change their orders. If money floods onto one selection, the price on that selection shortens and the prices elsewhere in the market drift. Those movements are the raw material of trading.

It also means the money in a market comes from other participants. Collectively, exchange traders cannot win, because commission is deducted from the winners. It is a negative-sum environment for the group, whatever the individual results.

Back and lay, properly defined

Backing is the familiar side. A back bet at odds of 4.0 with a £100 stake wins £300 if the selection wins and loses £100 if it does not.

Laying is the bookmaker’s side. Laying a selection means accepting someone else’s back bet. A lay of £100 at odds of 4.0 wins £100 if the selection loses, and loses £300 if it wins. That £300 is the liability, and the exchange reserves it from the account balance as soon as the bet is matched.

The liability on a lay is calculated as stake × (odds − 1). At 4.0, a £100 lay carries £300 of liability. At 1.20, a £100 lay carries only £20. At 15.0, it carries £1,400. This asymmetry catches out newcomers more than anything else in the mechanics: a lay at long odds risks a lot to win a little.

Backing is a bet that a price will shorten or that an outcome will happen. Laying is a bet that a price will drift or that an outcome will not happen.

Why a position can be closed before the event ends

A trader who has backed a selection at 4.0 holds a contract worth £300 of potential profit against £100 of risk. If the price then shortens to 3.0, that same exposure could now only be obtained for less. The original position has gained value.

The trader does not have to wait for the match to find out. By laying the same selection at the new, shorter price, the two bets sit on the same market and offset each other. The result is a set of outcomes that no longer depends on whether the selection wins.

This is where trading separates from betting. A bettor holds an opinion about the result. A trader holds an opinion about the price, and can be right about the price while being wrong about the result — or vice versa.

Working through a green-up, step by step

“Greening up” means closing a position so that the same profit is realised whichever way the event goes. The arithmetic is worth doing slowly, because once it clicks the whole concept becomes obvious.

The starting position

A £100 back bet is matched on a selection at odds of 4.0.

The price moves

The selection attracts money and the price shortens to 3.0. The question is: what lay stake makes both outcomes identical?

Let the lay stake be X, placed at odds of 3.0, so the liability is 2X.

If the selection wins, the back bet pays £300 and the lay bet loses its liability: 300 − 2X

If the selection loses, the back bet loses £100 and the lay bet collects its stake: X − 100

Setting the two equal:

300 − 2X = X − 100 400 = 3X X = £133.33

Checking both outcomes

The same figure, give or take a penny of rounding, regardless of the result. The trade produced roughly £33 on a £100 stake because the price moved from 4.0 to 3.0.

The shortcut

The full algebra is not needed every time. The equalising lay stake is:

(back stake × back odds) ÷ lay odds

Here: (100 × 4.0) ÷ 3.0 = £133.33. The same formula works in reverse for a position opened with a lay: (lay stake × lay odds) ÷ back odds gives the back stake required to close it.

Exchange interfaces do this automatically with a green-up button. Understanding the arithmetic behind the button matters, because it makes clear where the money actually comes from — the difference between two prices, scaled by the size of the position.

When the trade goes wrong

The same maths runs in the other direction. Suppose the £100 back at 4.0 drifts to 5.0 instead.

Equalising lay stake: (100 × 4.0) ÷ 5.0 = £80.

A loss of £20 locked in on both outcomes. This is the part that beginners tend to skip. Greening up is not a rescue mechanism; it is a way of fixing a result, and the result can be negative. A trader who only closes winners and lets losers run to the event will accumulate small gains and occasional full-stake losses.

What commission does to the numbers

Betfair Exchange charges commission on net winnings in a market, typically in the range of 2% to 5% depending on the market and jurisdiction. It is levied per market, on the net position, not on turnover.

Applied to the green-up above: £33.33 of profit at 5% commission becomes £31.67. At 2%, £32.66.

Three practical consequences follow.

First, a trade that finishes exactly level pays nothing. Commission only bites on profit.

Second, small edges shrink noticeably. A trader scalping one tick at a time is handing over a slice of every winning market, while losing markets attract no rebate. The break-even hit rate is therefore higher than the raw odds suggest.

Third, because commission is charged on the net market position, a losing trade earlier in the same market offsets a later winning one before commission is calculated.

Liquidity and the spread, in practice

Liquidity is the money available to be matched at each price. A Premier League match odds market may hold tens of thousands of pounds at the front of the queue and turn over millions in play. A women’s second-tier tennis match at 3am might have £40 available and a price that jumps three ticks at a time.

Liquidity determines whether a position can be closed at all. A trade that cannot be exited is not a trade; it is a bet.

The spread is the gap between the best available back price and the best available lay price. On a liquid football market it might be 3.00 to 3.05. On a thin market it might be 3.00 to 3.40.

The spread is a real cost. Consider a £100 back matched at 3.00, closed immediately by laying at 3.05:

Lay stake: (100 × 3.00) ÷ 3.05 = £98.36

Opening and instantly closing a position costs £1.64 per £100 staked at that spread. Nothing happened, no price moved, and the position is down. On wider markets the same round trip can cost several per cent.

This is why liquid markets dominate serious trading activity, and why thin markets look deceptively attractive on a screen full of apparent value.

The mechanics are not the skill

Everything above can be learned in an afternoon. The arithmetic is fixed, the buttons are labelled, and the software calculates stakes automatically.

What cannot be learned in an afternoon is judgement about where prices are going: reading how a market absorbs money, recognising when a drift is information rather than noise, understanding how in-play prices respond to a shot on target versus a corner, knowing when a market’s liquidity is about to evaporate.

The honest position is this: the large majority of people who attempt exchange trading lose money over time. Commission, spreads and the presence of well-capitalised, automated participants make it a demanding environment. Consistent profitability requires a genuine, repeatable edge, and most participants do not have one. Small sample sizes disguise this for a while — a good week proves very little.

Anyone studying the field is better served by tracking every trade, measuring results across hundreds of markets rather than dozens — our own attempt to do exactly that is reported in what pre-match odds movement actually tells you — and treating early losses as the cost of tuition rather than as bad luck. Practising the mechanics with trivial stakes until the arithmetic is second nature costs almost nothing and removes one category of error entirely.

The green-up calculation above is the foundation. Reading the market is the actual work, and it takes far longer.

This article is provided for informational purposes only. It does not constitute betting or investment advice, and no outcome or return is implied or guaranteed. Gambling can be addictive and carries a risk of financial loss. Strictly 18+. Anyone concerned about their gambling should seek support from a recognised assistance service.