Position sizing determines how many shares, units, lots or contracts you trade. On a phone, the calculation matters just as much as it does on a desktop. The difference is the workflow: a small order ticket, a saved quantity or a misplaced decimal can turn a sensible trade into an oversized one.
The practical approach is to set a cash risk budget, identify your entry and exit levels, then calculate the quantity. Buying power comes later. This guide focuses on that calculation and the checks needed before submitting an order, rather than the broader rules covered in mobile trading risk management.
Position Size Is Not the Same as Money at Risk
A position has several different values. Mixing them up produces bad sizing decisions, even when the arithmetic looks tidy.
Position value is the market exposure you hold. Buying 50 shares at $40 creates a $2,000 position. Planned trade risk is the estimated loss between your entry and intended exit, plus relevant costs. If you plan to exit those shares at $39, the price risk is $50 before costs, assuming execution at that price.
Margin is a separate funding requirement. It is not a measure of the loss you have chosen to accept. In US securities margin accounts, losses can exceed the money deposited, and firms can sell assets without contacting the customer first. Those risks appear in FINRA’s required margin disclosure statement.
Treat an app’s “available to trade” figure as a funding constraint, not a spending target. A broker accepting an order does not make its size appropriate for your account.
The Position Sizing Formula
The basic relationship is:
Position size = cash risk budget ÷ estimated loss per unit
For shares quoted in your account currency, estimated price loss per share is the absolute difference between entry and stop price. Choose the stop from the trade setup, then calculate quantity from the risk budget. Do not move the stop closer simply to afford more shares. These two inputs—stop location and acceptable account risk—form the basis of CME Group’s position sizing method.
For a more useful calculation, include costs:
Quantity = (cash risk budget − fixed trade costs) ÷ (price risk per unit + variable costs per unit + execution allowance per unit)
Round the result down to the permitted quantity increment. Then check that you can fund the position and that it fits your remaining exposure limits. The calculation gives a ceiling under your assumptions, not an instruction to use every available unit.
Set the Risk Budget Before Opening the Order Ticket
A percentage based budget starts with the account equity you have chosen as your sizing base. For example, $10,000 multiplied by 0.5% gives a $50 risk budget. The percentage is illustrative, not a recommendation or a safety threshold.
Choose a budget that fits your finances, strategy and tolerance for losing trades. Keep living expenses and emergency savings outside that decision. For a repeatable routine, record which equity figure you use and when you update it. Do not alternate between account balance, buying power and equity depending on which produces the largest trade.
A Worked Mobile Stock Trading Example
Suppose a hypothetical account has $10,000 in equity and a $50 trade risk budget. The planned purchase price is $40, with a stop at $39.20. Assume $2 in fixed entry and exit costs, plus a $0.05 per share allowance for execution uncertainty.
| Input | Calculation | Result |
|---|---|---|
| Cash risk budget | $10,000 × 0.005 | $50 |
| Price risk per share | $40 − $39.20 | $0.80 |
| Budget after fixed costs | $50 − $2 | $48 |
| Adjusted risk per share | $0.80 + $0.05 | $0.85 |
| Calculated quantity | $48 ÷ $0.85 | 56.47 shares |
| Whole share quantity | Round down | 56 shares |
The resulting position value is $2,240. Estimated loss under these assumptions is $49.60: 56 multiplied by $0.85, plus $2. If the account cannot fund $2,240, reduce the quantity further. Risk sizing does not override cash availability.
Keep the final figures together in a note or worksheet: entry $40, stop $39.20, quantity 56, estimated risk $49.60. That short record is easier to compare with an order preview than a calculator screen showing an unexplained “56.47”.
Allow for Costs Without Counting Them Twice
A bare entry minus stop calculation leaves no room for trading costs. Include commissions and other applicable charges for both opening and closing the position. If a charge depends on quantity, include it per unit rather than treating it as fixed.
Handle the spread consistently. When your calculation already uses the expected executable entry and exit prices, adding the full spread again can double count it. When you start from a chart price that is not executable, adjust the assumptions to reflect the relevant bid and ask.
An execution allowance is a planning buffer, not insurance. In the example above, $0.05 per share was an invented assumption to demonstrate the method. It was not a forecast of slippage or a maximum possible deviation.
A Stop Price Does Not Guarantee the Planned Loss
For stocks, a conventional stop order becomes a market order when triggered, and its execution price can differ substantially from the stop price. A stop limit order controls the acceptable price but may remain unfilled. The SEC bulletin on stop and stop limit orders sets out this distinction.
Suppose the 56 shares in the example were instead sold at $38.50. The price loss from a $40 entry would be $84 before charges, already above the $50 budget. Position sizing estimates risk under stated conditions; it does not turn an uncertain exit into a guaranteed one.
Adjust the Calculation to the Instrument
Shares and Fractional Quantities
For ordinary shares, divide the available risk budget by the adjusted loss per share. If the ticket permits fractional quantities, round down to its supported increment rather than automatically rounding to a whole share.
Check the field label carefully. An order for “$500” is not an order for “500 shares”. Before confirming, compare the displayed quantity, estimated order value and intended stop. Do not assume fractional orders support the same exit instructions as whole share orders; check the available order settings.
Forex Lots
For forex, express the stop distance in pips and convert the pip value into your account currency:
Lots = available risk budget ÷ (stop distance in pips × pip value per lot)
Assume, purely for illustration, that one standard lot has a pip value of $10 in the account currency. A 20 pip stop represents $200 of price risk per standard lot. A $40 budget therefore permits 0.20 lots before costs.
Do not reuse that assumed pip value across currency pairs. Verify the pair, account currency, contract size and volume increment. The related guide to forex lot size and margin calculators covers those inputs in more detail.
Futures Contracts
For futures, translate the stop distance into ticks, then multiply by the monetary value of each tick. Multiply that result by contract quantity to obtain price risk. Contract size and tick value must match the product being traded; CME Group’s futures profit and loss calculation demonstrates the relationship.
Using hypothetical contract terms, a 12 tick stop with a $1.25 tick value creates $15 of price risk per contract. If assumed entry and exit charges total $2 per contract, the estimated risk becomes $17. A $50 budget permits two contracts, not three, before any further execution allowance.
If one contract exceeds the budget, the calculated size is zero. Check whether a suitable smaller contract exists, or pass on the trade. Moving the stop to an arbitrary level just to squeeze in one contract defeats the calculation.
Use a Repeatable Phone Workflow
Prepare the calculation before opening the live ticket. A saved worksheet should contain account equity, risk budget, instrument, account currency, entry, stop, unit value, costs and rounded quantity. Label every input. “Value” alone is not a helpful label when it might mean dollars, pips or contracts.
Keep manual entry to a minimum, but do not trust saved inputs without checking them. A calculator can produce a perfectly accurate answer for the wrong symbol. The same goes for a previous trade’s stop distance.
- Calculate: enter the proposed trade details and round quantity down.
- Compare: match the worksheet against the app’s symbol, direction, quantity and order value.
- Confirm: review entry and exit instructions, submit, then inspect the resulting order and position records.
Read decimals deliberately. A ticket showing 1.00 lots is ten times the volume of 0.10 lots. The decimal point does not care how good the trade idea is.
If the expected entry changes before submission, recalculate. With a stop fixed at $39.20, moving a planned stock entry from $40 to $40.20 increases price risk per share from $0.80 to $1.00. Keeping the old quantity would increase the planned loss.
Fit Each Trade Inside the Remaining Risk Budget
A trade can satisfy its individual sizing rule and still be too large alongside existing positions. Add the planned losses of open trades before accepting another one. Also consider whether several positions depend on the same market move.
For example, three hypothetical trades with $40 of planned risk each create $120 of combined planned risk. If all three express essentially the same bullish view, do not treat them as three unrelated decisions. Set a combined budget for that shared exposure before allocating quantities.
The remaining daily budget can create another constraint. Suppose your daily loss threshold is $150, realised losses are $90 and existing positions carry $40 of further planned risk. Under a rule that reserves both amounts, only $20 remains for a new trade. A normal $50 allocation no longer fits. Define that treatment in advance through your daily loss limit rules, rather than improvising on the order screen.
Treat Every Addition as a New Sizing Decision
Adding to a position changes total quantity, average entry and loss at the intended exit. Recalculate the combined position before submitting the extra order. The relevant question is not whether the second order looks small; it is what the entire position could lose under the revised plan.
The same discipline applies after a partial exit. Check that the remaining protective order matches the remaining position. Do not assume an old order quantity is still correct because the trade has become profitable.
Review the Quantity After Execution
Compare the actual fill with the planned entry. A different fill can change risk even when quantity stays the same. For a partial fill, calculate risk from the filled quantity while also accounting for any unfilled order that could still execute.
If you cannot establish what filled, pause new orders until the position and order records are clear. Repeatedly tapping submit is not a reconciliation method.
Record planned quantity, filled quantity, entry, stop, estimated risk and eventual costs in your phone trading journal. Review mismatches rather than judging the sizing process only by whether a trade made money. An oversized winner is still a sizing error.
The final rule is straightforward: calculate first, round down, verify the ticket and check the fill. If the smallest permitted position exceeds your budget, do not force the trade. A phone makes order entry convenient; it does not make an oversized position more affordable.