Fixed-Fractional vs Fixed-Risk on a Trailing DD

Two traders with identical strategies can end an evaluation in opposite places purely because of how they translate “risk” into size. On a static drawdown the difference is cosmetic. On a trailing drawdown — where the floor chases your equity higher — the choice between percent-of-equity and fixed-dollar sizing decides whether your buffer survives a normal give-back.

Getting this right isn’t a preference. It’s the difference between a rule that quietly protects you and one that quietly sets you up to breach a green account.

Two ways to define “risk”

Fixed-fractional (percent-of-equity) sizing risks a set percentage of your current account on every trade. As equity rises, your dollar risk rises; as equity falls, it shrinks. Size is always relative to what you have right now.
Fixed-risk (fixed-dollar) sizing risks the same dollar amount on every trade regardless of where the account sits — a flat number you chose at the start.

Both are legitimate. Both keep a swing book sane. The catch is how each interacts with a floor that moves, and here they behave almost like opposites. Our trailing-drawdown explainer covers exactly how the floor tracks your equity — read it first if the mechanic is new, because the sizing choice only makes sense once you can see the floor moving.

Percent sizing shrinks risk into a drawdown

Fixed-fractional sizing has a built-in safety property: it de-risks automatically when you’re losing. Each loss lowers your equity, and the next trade — being a percentage of that smaller number — risks fewer dollars. You throttle down without deciding to.

On a trailing drawdown this is exactly the reflex you want. When you’re giving back profit, the floor is still up near your recent peak, so your buffer is thin precisely when a fixed-dollar loss would hurt most. Percent sizing tapers your bets as the account bleeds, stretching a losing streak across more trades and keeping the floor further away. The cost is on the upside — as you profit, your dollar risk grows, which can bump into a consistency rule if you’re not watching the shape of your days.

Fixed-dollar sizing can over-risk after a give-back

Fixed-risk sizing does the opposite of what a trailing floor wants. Because the dollar amount never changes, a loss doesn’t shrink your next bet — you keep firing the same size while your buffer to the trailing floor gets thinner with every give-back.

Picture a run where you push the account up, the floor trails up behind you, and then you hand some profit back. Your equity has fallen toward the floor, but your fixed-dollar risk hasn’t moved an inch. The same loss that was a small fraction of a fat account is now a large fraction of your remaining room. Fixed-dollar sizing feels disciplined — it’s the same number every time — but under a trailing floor that constancy is the trap. A drawdown calculator makes this visible: plug in your live distance to the floor and watch how a flat dollar risk consumes a growing share of it as the buffer narrows.

Sizing off distance, not balance

The insight that resolves the whole debate: on a trailing drawdown, your real risk budget isn’t your balance and it isn’t a percentage of it — it’s your live distance to the current floor. That number moves independently of your balance as the floor trails, and it’s the only quantity that actually governs whether you breach.

Recompute your distance-to-floor before each session, not just at the open.
As the floor trails up toward your equity, your effective buffer shrinks even if your balance is flat — so your size should shrink too.
Once a trailing floor locks (many freeze at the starting balance after you’ve earned a set cushion), banked profit finally becomes real, and you can size more freely within your rule.

Anchoring to yesterday’s floor is how traders breach a profitable account. Size against today’s, every day.

A hybrid: percent of remaining buffer

The cleanest method blends both approaches: risk a fixed percentage of your remaining buffer — your distance to the floor — rather than a percentage of account equity or a flat dollar figure.

This inherits the good behavior of both. Like percent sizing, it de-risks automatically when you’re losing, because a smaller buffer produces smaller bets. Unlike account-percent sizing, it’s calibrated to the number that actually kills you — the floor — instead of a balance the trailing rule mostly ignores. When your buffer is fat, you have room to work; when it thins, you’re pulled down proportionally without needing willpower.

Reason about it in R-multiples: let your buffer hold a fixed number of R at all times — say, a buffer that can absorb twenty consecutive 1R losses — and back out the dollar risk per trade from there. A position size calculator then converts that risk into lots or contracts at your live stop distance, so the size updates itself as both the buffer and the stop move.

Choosing per style and locking it in

There’s no universal winner — there’s a right answer for your style, chosen in advance:

Trend and swing traders riding runners usually want percent-of-buffer sizing, so a deep drawdown throttles them down before it becomes a breach.
Scalpers and high-frequency traders with tight, uniform stops can run closer to fixed-risk, because their stops don’t vary much — but they should still cut size hard when the buffer thins.
Everyone should decide the rule while calm and never renegotiate it mid-drawdown, which is exactly when the temptation to “make it back” corrupts good sizing.

The rule only protects you if you can’t override it in a weak moment. Shibiki enforces maximum size and maximum loss per trade at the broker EA, so an order that breaks your sizing rule is refused rather than filled — and its auto-journaling logs every fill so you can see, honestly, whether your live sizing matched the plan or drifted. If you’re running the same approach across several evaluations, copying across prop accounts applies the identical rule everywhere, so one correct decision protects every account at once.