Discount Calculator

A discount calculator applies any of ten discount structures, percent off, fixed amount off, buy-X-get-Y, Nth-item percent off, multi-buy (N for M), N-for-fixed-price, bundle, and sequential double or triple stacking, to a subtotal, respecting an optional discount cap and price floor, and handling tax as configured. Each discount type ships with an example illustrating its structure.

Pricing runs from a single base unit price and item count, or from a cart table holding several products with independent prices and quantities. Tax can be marked as already included in the listed prices, with a configurable sales tax rate, and applied before or after the discount. A discount cap (as a percent or a flat dollar amount) and a price floor (as a percent of list price or a dollar amount per unit) constrain how far the discount can reduce the total. Results report the subtotal, the discount amount and percent, tax, the total after tax, the effective per-unit price, and the details of how the deal was applied.

Use the following plain-text formulas and logic for quick reference:

• Percent off: discounted_subtotal = subtotal × (1 − p), where p is percent/100.
• Fixed amount off: discounted_subtotal = max(0, subtotal − A).
• Buy X get Y (discount d% on Y): set_size = X + Y; eligible_sets = floor(Q / set_size); discounted_units = eligible_sets × Y; savings = sum_of_highest_prices_for(discounted_units) × d.
• Nth-item percent off: sort all unit prices descending; apply percent r to every Nth item starting at index N; savings = sum(selected_unit_prices) × r.
• Multi-buy (N for M): eligible_sets = floor(Q / N); free_units = eligible_sets × (N − M); savings = sum_of_highest_prices_for(free_units).
• N for fixed price ($B): eligible_sets = floor(Q / N); bundled_total = eligible_sets × B; unbundled_qty = Q mod N; unbundled_total = sum_of_highest_prices_for(unbundled_qty); savings = (normal_total − (bundled_total + unbundled_total)).
• Bundle (size S): eligible_sets = floor(Q / S); apply benefit as configured:
  • Percent off on entire bundle: savings = bundle_value × percent.
  • Percent off on cheapest or most expensive items within bundles: savings = sum(target_items) × percent.
  • Amount off: savings = amount × eligible_sets.
  • Fixed bundle price: savings = (bundle_list_total − fixed_price) × eligible_sets.
• Double/Triple discount (sequential): apply each discount in order to the remaining subtotal, not additively. Example: after percent a% then amount $b then percent c%:
  step1 = subtotal × (1 − a); step2 = max(0, step1 − b); step3 = step2 × (1 − c).
• Price target: required_discount = max(0, subtotal − target); required_percent = (required_discount ÷ subtotal) × 100.
• Cap: max_savings_% = subtotal × cap%; max_savings_$ = cap_amount; savings = min(savings, cap).
• Price floor:
  floor_%: min_price = subtotal × floor%; max_savings = subtotal − min_price;
  floor_$ per unit: max_savings = subtotal − (floor_per_unit × total_units);
  savings = min(savings, max_savings).
• Tax:
  • Discount before tax: tax = discounted_subtotal × rate; total = discounted_subtotal + tax.
  • Tax included in prices: pre_tax = discounted_subtotal ÷ (1 + rate); tax = pre_tax × rate; total = discounted_subtotal.
• Per-unit metrics: effective_price = discounted_subtotal ÷ total_units; savings_per_unit = (subtotal ÷ total_units) − effective_price.
• Tie-breaking when prices vary: when deals specify free or discounted units, higher-priced units are prioritized first for customer-friendly savings.

The formulas above cover retail and ecommerce scenarios where promotions must be compared, purchases planned, and pricing rules validated.

• Comparing a "25% off" coupon against "3 for 2" finds the lower effective unit price.
• Evaluating "Buy 2, get 1 free" with mixed prices shows how savings change by item mix.
• Fitting a cart to a target subtotal (for free shipping or a budget limit) computes the required discount percent.
• Enforcing price floors or caps simulates store policy and confirms compliant totals.
• Including sales tax accurately tests "discount before tax" versus "tax-included" pricing.
• Modeling bundle offers like "any 3 for $10" or "20% off the cheapest 3 in a set of 5."
• Stacking sequential discounts reflects loyalty, coupon, and clearance reductions applied in order.