ForexFin
ForexFin
The largest risk per trade your win rate and average R can support, with half- and quarter-Kelly.
At a 55% win rate and a 1.5 average R-multiple, full Kelly is 25.00% of bankroll — edge = 1.5 × 0.55 − 0.45 = +0.375 R per trade, and f* = 0.375 ÷ 1.5. Almost no one trades full Kelly; the standard is half-Kelly (12.50%) or quarter-Kelly (6.25%), which keep most of the long-run growth at a fraction of the drawdown. A negative edge returns a negative fraction — don't take the trade.
Enter your win rate and your average R-multiple (average win ÷ average loss). The calculator returns your edge and the full Kelly fraction; add an account balance to see the amount to risk at full, half and quarter Kelly.
The Kelly criterion gives the bet size that maximizes the long-run growth rate of your bankroll given a known edge:
f* = (b·p − q) ÷ b where b = R-multiple, p = win rate, q = 1 − p
The same formula in trading terms: f* = W − (1 − W) ÷ R, where W is your win rate and R your average win ÷ average loss. At a 55% win rate and 1.5 R: 0.55 − 0.45 ÷ 1.5 = 0.25, so full Kelly is 25% of the account.
If the formula returns a negative number, your edge is negative — don't take the trade.
Full Kelly maximises long-run growth only if you know your true edge exactly. In real trading you don't — your win rate and R-multiple are noisy estimates from a finite sample. Full Kelly on an overestimated edge leads to deep drawdowns.
Half Kelly retains ~75% of the growth rate with roughly half the volatility. Quarter Kelly is common for discretionary traders and fund managers who want smoother equity curves.
Quarter Kelly keeps about 44% of the growth rate. Past twice the Kelly fraction, expected growth turns negative even with a positive edge.
Use your own closed trades. The ForexFin trade journal shows win rate, average win and average loss; if you risk a similar amount on each trade, average win ÷ average loss is the R-multiple to enter here. Under about 50 trades, both figures are noisy, so stay at quarter Kelly or below.
ForexFin's analytics work on your trade history: import MT4/MT5 by CSV, score every trade that has a stop in R, and see which setups, pairs and sessions carry your results. The journal is free; Trader adds behaviour flags and written retrospectives.