Sizing one trade is arithmetic. Sizing the same trade across six accounts of different balances and drawdown models is where most multi-account traders quietly break their own rules — usually by starting from the wrong number.
Start from risk-per-account, not lots
The mistake is to pick a lot size or contract count on the master and copy it everywhere. That fixes the size and lets the risk float — which is backwards. On accounts of different balances, the same lot size is a different percentage of each account, so “identical copies” put wildly different risk on each book.
Start from the other end. For every account, decide risk-per-account first — the currency amount you’re willing to lose if the stop is hit — and let the size fall out of it:
- Pick a fixed fraction of each account’s balance as the per-trade risk (many prop traders keep this well under the firm’s daily loss limit, then confirm the actual limit with the firm).
- That gives you a currency risk budget per account, not a lot size.
- Size is whatever puts exactly that many currency units at risk given the stop.
Now every account risks the same proportion of itself on the same idea, which is what “copying” should have meant all along. The position size calculator runs this direction — risk budget in, size out — for each account.
Stop distance drives every account’s size
The bridge between “I’ll risk $200 on this account” and “so I trade X lots or Y contracts” is the stop distance. It’s the same for every account on the same trade, because it’s a property of the market, not the account.
The core relationship:
Size = Risk budget ÷ (Stop distance × Value per unit of move)
- Risk budget is your per-account currency amount (different per account).
- Stop distance is entry-to-stop, in points, ticks, or pips (identical across accounts).
- Value per unit of move is what one point/tick/pip is worth for one lot or contract (a property of the instrument).
Because stop distance is shared, the only thing that changes each account’s size is its risk budget. Widen the stop and every account’s size shrinks in lockstep; that’s correct — a wider stop means fewer units to keep the same money at risk. The lot size calculator and risk-reward calculator let you sanity-check the stop and target before the size math even starts.
Normalizing to one R across the portfolio
Here’s the move that makes multi-account trading legible: express everything in R, where 1R is the risk budget on that account. If Account A risks $150 and Account B risks $400 on the same trade, both are risking exactly 1R — the dollar amounts differ, the risk unit doesn’t.
This matters because it lets you reason about the whole portfolio as one number:
- A trade that returns +2R returned +2R on every account, regardless of balance — $300 on A, $800 on B.
- Your portfolio result for the day is just the sum of R across accounts, which you can compare trade-to-trade even as balances drift.
- An edge measured in R is portable: it doesn’t get distorted by which account happened to be bigger that week.
If R is new to you, the R-multiple explainer is the foundation — everything in multi-account sizing gets simpler once every account speaks the same unit.
Rounding rules that break proportionality
Then reality intrudes. Forex lets you size to two decimals; futures only trade whole contracts, and even forex has a minimum increment. The instant you round, proportionality cracks.
Worked example — same trade, three accounts, sizing to whole contracts:
| Account | Risk budget | Ideal contracts | Rounded | Actual risk |
|---|---|---|---|---|
| A | $600 | 3.0 | 3 | 1.00R |
| B | $400 | 2.0 | 2 | 1.00R |
| C | $250 | 1.25 | 1 | 0.80R |
Account C wanted 1.25 contracts, can only trade 1, and is now under-risking at 0.8R. Round it up to 2 instead and it jumps to 1.6R — a 60% overshoot on the account that can least afford it. Neither is “wrong”; both are the cost of integer sizing.
The practical rules:
- Round down on trailing-drawdown accounts. Under-risking is survivable; over-risking near a trailing line is how accounts die.
- Trade micros where you can — ten times the granularity means rounding error shrinks by an order of magnitude.
- Accept that small accounts copy roughly. An account that ideally wants a fraction of one contract simply can’t mirror precisely, and pretending otherwise hides real risk drift.
A reusable sizing table
Rather than recompute per trade, build a table once per session and reuse it. For each account, pre-store its risk budget (currency and R). Then each trade needs only one live input — the stop distance — to produce every account’s size.
A workable layout to keep beside you:
- Per account: balance, risk-per-trade fraction, resulting risk budget in currency, and 1R in currency.
- Per trade: instrument, stop distance, value per unit of move.
- Computed: ideal size, rounded size, and the actual R each account ends up carrying after rounding.
That last column is the honest one — it tells you, before you fire, exactly how far each account has drifted from the plan. Most traders never look at it.
This is the arithmetic Shibiki keeps for you automatically: it captures each account’s fills, converts every result to R so a mixed-balance portfolio reads as one clean series, tracks live edge health per strategy with a Wilson confidence interval so you don’t scale a mirage, and enforces hard risk limits at the broker so a rounding overshoot can’t push an account past its line. The math above is the point — the tool just stops it from silently rotting between trades.
Related: Position size calculator · R-multiple explained · Risk-reward calculator