Position sizing across multiple accounts

Plenty of swing traders run more than one account — an IRA for the long-hold tax shelter, a margin account for faster trades, maybe a small experimental account on the side. The math of position sizing doesn't change with two accounts, but the bookkeeping does, and that's where most sizing mistakes come from. This guide covers the approach that keeps risk consistent everywhere.

The one rule: risk percent is per account

Position sizing starts from a simple formula: shares = account risk ($) ÷ stop size ($/share), where stop size is the distance between your entry and your stop loss. The risk dollars come from a fixed percent of the account — commonly 0.25%, 0.5%, or 1%.

With multiple accounts, apply the percent to each account's own balance, never to the combined total. A 0.5% risk trade in a $40,000 IRA risks $200; the same setup in a $10,000 margin account risks $50. The share counts differ, but each account carries the same proportional risk — which is what makes drawdowns survivable and results comparable across accounts.

A worked example

Say a stock breaks out at $50 and the low of day — your stop — is $48. Stop size is $2/share. At 0.5% risk:

Both positions lose exactly half a percent of their account if the stop hits. Notice the cost column: the IRA position ties up $5,000 of a $40,000 account (12.5% of buying power) even though only $200 is at risk. Risk and exposure are different numbers — track both.

Why not size off the combined total?

Sizing off combined balances breaks down in practice for three reasons:

• Cash isn't fungible across accounts. You can't move IRA money to cover a margin position without tax consequences, so a "portfolio-sized" position can exceed what one account can actually hold.
• Account rules differ. Settlement times, margin, and day-trade limits vary. A size that's fine in one account may be unplaceable in the other.
• Performance gets muddy. Per-account risk keeps each account's R-multiples honest: a +2R trade means the same thing everywhere.

Sizing off a low-of-day stop

Momentum and breakout traders often place the stop at the low of day (LoD). The logic is invalidation, not magic: if a stock breaks out and then trades back below the lowest price of the session, the move that justified the entry has usually failed, so that price is a natural "I'm wrong" line.

What matters for sizing is that the LoD is a moving stop until you enter. Stop size is entry − stop, so every tick in the day's low changes your share count:

Same $200 of risk in every row — a tighter stop simply buys more shares, a wider one fewer. That's why sizing off a stale low is the classic fat-finger: if the low moved while you were doing arithmetic, your risk is no longer what you planned.

Two boundaries worth being honest about. First, once you're in and price tags the stop, you're out — your broker's stop order handles that. A sizing tool's job is before the trade (how many shares) and during it (how much is at risk if every stop hits); it is not a stop-out alert. Second, with multiple accounts the LoD stop distance is the same for every account — only the share counts differ, scaled to each balance, and portfolio heat still sums the dollar risk across all of them.

Risk Calc's free tier locks the stop to the live low of day and re-sizes all three risk tiers in real time on one account; Pro does the same across every account you trade at once.

Track portfolio heat separately

The number that should span accounts is portfolio heat: the total dollars you lose if every open stop hits today. Sum (entry − current stop) × shares across all open positions in all accounts. Many swing traders cap total heat around 2–6% of combined balances — beyond that, a bad day compounds into a bad month. When a trade goes well and you move its stop to break-even, that position drops out of the heat number entirely, freeing risk budget for the next setup.

The bookkeeping problem, solved

Doing this by hand every morning — three risk tiers × N accounts, recomputed as the low of day moves — is exactly the kind of arithmetic that produces fat-finger sizing errors at the open. Risk Calc does it live: name each account with its balance, and every risk tier shows the share count per account against the streaming price and a stop locked to the low of day. The position ledger then tracks open risk per account and portfolio heat as your stops move.

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Risk Calc is an educational position-sizing tool. This guide is informational only — not financial, investment, or trading advice, and not a recommendation to buy or sell any security. Trading involves risk of loss.