Define the profit pool the promotion must protect
Start with the regular pre-tax selling price. Fixed variable cost means the order-level costs that do not fall just because the price is discounted: product cost, pick and pack, packaging, a fixed shipping subsidy and other per-order charges. The variable fee rate is reserved for costs that move with charged revenue, such as a percentage payment or marketplace fee. The result is contribution before advertising, returns, tax and fixed overhead unless you deliberately include another order-level cost.
Use one comparable period for current order volume, such as a week or 30 days. The tool uses that count to translate the percentage hurdle into a target number of orders. The required percentage itself is driven by unit economics, so changing 100 current orders to 1,000 changes the absolute target but not the required lift percentage.
The required volume increase formula
Original contribution per order = regular price × (1 − variable fee rate) − fixed variable cost. Discounted contribution per order uses the discounted price in the same formula. Required order multiplier = original contribution per order ÷ discounted contribution per order. Required volume increase = (required multiplier − 1) × 100%. The calculation preserves total contribution dollars, not revenue or contribution margin percentage.
For a $50 product with $30 of fixed variable cost, no percentage fee and 100 current orders, original contribution is $20 per order and $2,000 in total. At 15% off, price falls to $42.50 and contribution falls to $12.50. The required multiplier is $20 ÷ $12.50 = 1.6, so the promotion needs 160 orders: 60 more orders, or a 60% lift, just to return to $2,000 of contribution.
Why a modest discount can demand a much larger sales lift
The discount is applied to revenue, while the loss is absorbed by the smaller contribution left after costs. In the same $50 and $30 example, a 20% discount removes $10 of price but cuts per-order contribution from $20 to $10. Each promoted order contributes only half as much, so order volume must double—a 100% increase—to preserve the original total.
The break-even discount ceiling marks the point where discounted contribution per order reaches zero under the entered cost model. At that point, selling more units adds no contribution; beyond it, every additional order increases the loss. Treat this ceiling as a mathematical boundary, not a recommended promotion depth, because the business still needs room for customer acquisition, returns and overhead.
Handle payment fees and changing basket costs consistently
Put percentage-of-revenue charges in the variable fee field so they fall as the discounted price falls. Put cents-per-transaction charges in fixed variable cost. If a provider charges both, split the fee across both inputs. This keeps the before-and-after comparison more accurate than treating the whole payment fee as fixed.
The model assumes the promoted order contains the same unit and fixed variable cost as the regular order. A storewide code can change product mix, basket size, shipping weight and refund behavior. If the promotion applies to multi-item carts, model the expected promoted average order rather than a single SKU, and replace the cost inputs with the weighted costs of that basket.
Turn the hurdle into a promotion test plan
Record the baseline period, audience, traffic source, orders, charged revenue and contribution using the same cost definition as the calculator. Before launch, compare the required order target with available traffic. If a campaign needs 160 orders from the same 100-order traffic base, the conversion requirement is also 60% higher unless the promotion brings additional qualified visitors.
After the test, compare total contribution rather than celebrating revenue or order count alone. Track discount redemptions, conversion rate, average order value, new versus returning customers, refund rate and contribution after media spend. A promotion can miss the immediate hurdle but still have value through repeat purchases; that value should be measured from cohort data, not assumed in advance.
Know what the calculator does not predict
The calculator does not predict demand elasticity, conversion lift, incremental traffic or customer lifetime value. It answers the narrower question: if unit cost behavior stays as entered, how many orders would preserve the current total contribution? Whether the market will supply those orders requires a test or historical promotion data.
Taxes, returns, chargebacks, inventory constraints, overtime, tiered fulfillment fees and fixed campaign costs can change the result. Fixed overhead does not normally change with each order, but an incremental creative, influencer or email cost belongs in the final campaign evaluation. Recalculate whenever discount eligibility, product mix, fee structure or shipping policy changes.