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Unit sizing math

Lesson 11 · about 9 min

Spot crypto is the easiest market in the world to size correctly, because you can buy fractions of a unit and there is no contract multiplier. It is also the market where beginners most often size by feel ("I'll put in $500") instead of by arithmetic. This lesson is the arithmetic, and it assumes you know the fixed-fractional method from the risk management course.

The three numbers

  1. Dollars at risk. A fixed fraction of your account, typically 0.5% to 1%.
  2. Stop distance. Entry price minus stop price, in the quote currency.
  3. Units. Dollars at risk ÷ stop distance.

That is the whole method. Everything else is checking the answer against other limits.

Example 1: a clean BTC trade

Account $4,000. Risk 1% = $40. Entry $62,000, stop $60,500.

  • Stop distance = 62,000 − 60,500 = $1,500.
  • Units = 40 ÷ 1,500 = 0.02667 BTC.
  • Position value = 0.02667 × 62,000 = $1,653.
  • Loss at stop = 0.02667 × 1,500 = $40. Check.

Note that the position is 41% of the account. That is fine for spot with no leverage, provided your plan's single-position cap allows it. If the cap is 25%, take 0.01613 BTC ($1,000) and risk $24 instead. Never solve the conflict by moving the stop.

Example 2: an alt with slippage built in

Account $4,000. Risk 0.75% = $30. Entry $1.20, planned stop $1.12. You expect 1% slippage on entry and 2% on the stop because the book is thin.

  • Nominal stop distance = 1.20 − 1.12 = $0.08 (6.7%).
  • Slippage allowance = 1% + 2% = 3% of $1.20 = $0.036.
  • Effective stop distance = 0.08 + 0.036 = $0.116.
  • Units = 30 ÷ 0.116 = 258.6, round down to 258.
  • Position value = 258 × 1.20 = $310.
  • Worst realistic loss = 258 × 0.116 = $29.93.

Without the slippage allowance you would have bought 375 units and lost about $43 on a bad night, 45% more than planned. Small numbers, but the habit is what matters; the same error at ten times the size is $130 over budget.

Key idea: Units = dollars at risk ÷ (stop distance + expected slippage). Spot crypto lets you buy exactly that many units, so there is no excuse for a position that risks more than the plan says.

Example 3: quoting in a stablecoin

Everything above was in "dollars", but your account is probably in USDT or USDC. Treat one unit of a major stablecoin as one dollar for sizing, and remember two adjustments:

  • If your account is held in a stablecoin and it trades at $0.99 rather than $1.00 during stress, your real risk budget is 1% smaller. Ignorable most of the time, not during a de-peg.
  • If you trade a coin quoted in BTC (an ALT/BTC pair), the stop distance is in BTC and so is the risk. Convert: dollars at risk ÷ BTC price = BTC at risk, then divide by the stop distance in BTC.

Quick ALT/BTC example. Risk $30, BTC at $60,000, so BTC at risk = 0.0005 BTC. Entry 0.000200 BTC, stop 0.000185 BTC, distance 0.000015 BTC. Units = 0.0005 ÷ 0.000015 = 33.3, so 33 units.

Minimum order sizes and lot steps

Exchanges enforce a minimum order value (often $5 to $10) and a quantity step (for example, BTC in 0.00001 increments, some alts in whole units). Always round units down to the allowed step. If rounding down takes you below the minimum order, the trade is too small for your account at that stop; skip it rather than inflate it.

The depth cap

Module 1 introduced a second cap for alts: position value no larger than about 5% of the 2% bid depth. Run it after the risk-based size:

  • Example 2 gives a $310 position. If the 2% bid depth is $18,000, the cap is $900; the trade passes.
  • If the depth were $4,000, the cap is $200; take 166 units and risk about $19, or skip.

The smaller of the risk-based size and the depth cap is your size. Both limits exist because they fail in different ways: the first caps the loss if your stop fills, the second caps the loss if it cannot.

A one-line checklist

Before every spot entry: risk dollars, stop distance, slippage allowance, units, position value, cap check, step rounding. It takes thirty seconds and it is the only part of the trade you control completely. The position size calculator does the division; the discipline is doing it every time.

Try it: Account $2,500, risk 1%. Entry $0.85, stop $0.78, slippage allowance 2% of entry. Compute units, position value and worst-case loss (answers: effective distance $0.087, 287 units, $244 position, $24.97 loss). Then apply a depth cap with 2% depth of $3,000.

Recap

  • Units = dollars at risk ÷ stop distance; spot lets you buy exactly that fraction.
  • Add expected entry and exit slippage to the stop distance before dividing, especially on alts.
  • Check the result against your single-position cap and the depth cap; take the smallest.
  • Round units down to the exchange's step; if that breaks the minimum order, skip the trade.
  • For ALT/BTC pairs, convert risk to BTC first and size in BTC terms.

See it drawn

Original diagrams for the ideas on this page. Illustrative, not real market data.

How a position size is worked outAccount size, risk per trade and stop distance feed into one box giving the number of shares.ACCOUNT SIZE$25,000your capitalRISK PER TRADE1%of the accountSTOP DISTANCE$0.50entry to stopPOSITION SIZE500 sharesrisk budget: $25,000 × 1% = $250position size: $250 ÷ $0.50 = 500 shares
Working out a position size. Three numbers decide how big a trade is: the account, the share of it put at risk, and the distance from entry to stop. One percent of $25,000 is a $250 budget, and a $0.50 stop divides into that 500 times.
Slippage on a market orderA buy order clears four price levels, so the average price paid is worse than the price first quoted.Buy 1,000 shares at marketpricesell orders resting (bar length = size)20.04300 shares20.03200 shares20.01200 shares20.00300 sharesnothing resting at 20.02order sweeps up the bookaverage fill 20.02SLIPPAGE0.02 a share$20.00 in totalintended 20.00Each level fills at its own price; the average is what you really paid.
Slippage on a market order. You click at 20.00, but only 300 shares are resting there, so the rest of the order fills at 20.01, 20.03 and 20.04. The average price paid is 20.02, and that two-cent gap is slippage.