Sequential discounts multiply the remaining price
A 20% discount followed by another 10% discount leaves 0.80 × 0.90 = 0.72 of the original price, an effective 28% reduction. On $100 the final price before any tax is $72, not $70. Adding the two percentages assumes both reductions use the original price, which is a different offer.
A fixed-dollar coupon and a percentage discount also depend on their order. Confirm the retailer’s terms rather than assuming that every coupon can be combined. The stacked-percentage tool models the percentages you supply; it does not apply an unentered minimum spend, cap, category exclusion or coupon eligibility rule.
Tax belongs to an explicitly stated base
If the model applies an 8% tax after reducing a $100 price to $72, tax is $5.76 and the modeled total is $77.76. Applying the same tax to the original $100 produces a different result. The calculator describes its order of operations so you can compare it with the receipt; it does not establish which items, discounts or fees are legally taxable.
A receipt can round tax per item or across the whole taxable subtotal. Those approaches may differ by a cent even when the nominal rate is identical. For reconciliation, use the merchant’s actual subtotal and policy rather than treating an unrounded model as an official invoice.
Separate the tip rate from rounding the result
An 18% tip on a stated $47.50 base is $8.55 before a selected rounding rule. Rounding that tip to the nearest dollar gives $9 under a conventional midpoint rule, while rounding down gives $8. These choices deliberately change the payment; they are not extra precision in the percentage calculation.
Decide whether your supplied tip base includes tax or fees, and distinguish rounding the tip from rounding the final total. A calculator’s selected rule should be visible in the result. The arithmetic does not prescribe an appropriate tip amount or determine whether a charge already represents a gratuity.
Allocate cents so the contributions sum to the bill
Ten dollars split three ways has no equal two-decimal amount. A cent-preserving allocation is $3.34, $3.33 and $3.33. Displaying $3.33 for every person leaves one cent unpaid; displaying $3.34 for every person over-collects two cents. A deterministic remainder rule makes the difference explicit.
Equal splitting is a choice, not a claim that everyone ordered the same items. Unequal shares, reimbursements or item-by-item allocations are separate models. If the split total already includes tax and tip, avoid adding them again. Check the displayed total before dividing and verify that all allocated contributions sum to it exactly in integer cents.
Questions before you calculate
Is a 20% discount plus 10% another 30% off?
Not when each reduction applies to the remaining price. Multiply the remaining proportions: 0.80 × 0.90 = 0.72, or 28% off in total. Use addition only for an offer explicitly defined against the same original base.
Which person receives the extra cent?
The equal-split result uses a stated deterministic order for the remainder. The mathematical requirement is that the contributions sum to the rounded total, not that a particular person always pays the extra cent. The group can agree on a different allocation.
Are the default rates recommendations?
No. They are demonstration inputs used to illustrate the formulas. Replace them with the actual transaction rates and bases. Local tax treatment, retailer rules and any service-charge policy should be checked separately.