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Every calculator here is deterministic. No AI is inventing a number behind the curtain. This page shows the formula, the assumption it makes, and where platform-specific rules come from.
Formula: Book 1 royalty + the expected royalty from each later book, weighted by the probability that a Book 1 reader reaches it.
Assumption: because the simple tool asks for one read-through rate, it assumes the same conditional continuation rate between each book.
Why: this is expected value. A reader does not buy every later book, so later royalties are discounted by the chance of reaching them.
Formula: next-book readers ÷ previous-book readers.
Assumption: tracked reader cohorts measure continuation. Store sales totals give an estimated sales ratio; a matched reporting window does not ensure those sales came from the same readers.
Why: read-through is conditional. Book 2 → Book 3 asks what share of Book 2 readers continued, not what share of the original Book 1 audience remains.
Formula: maximum CPC = reader value × click-to-reader conversion rate.
Why: if one reader is worth $10 and 5% of clicks become readers, one click has $0.50 of expected reader value. Paying more than that loses money under those assumptions.
Not included: taxes, refunds, attribution gaps, creative costs or overhead unless they are already reflected in your reader value.
Formula: entered income − entered costs. Margin = remainder ÷ entered income.
Why: this is deliberately an operating view, not tax-accounting profit. If you do not enter a cost, the calculator does not pretend it exists.
Formula: KENP read × effective payout per KENP.
Why: KU payments come from the monthly KDP Select Global Fund based on pages read. The “per-page rate” is therefore an effective rate for a period, not a permanent fixed tariff.
Formula: royalty rate × tax-exclusive list price − printing cost.
Why: this is Amazon KDP’s documented print royalty formula. We keep the rate and print cost editable because the applicable rate and printing cost depend on price, marketplace and book specifications.
Rounding: the per-copy royalty is rounded to cents before multiplying by the number of copies in our total estimate.
Retailer side: we use the actual royalty per sale you enter.
Direct side: sale amount excluding customer tax − percentage fee on that amount − fixed fee − delivery cost.
Why: retailer ebook royalties can vary with VAT, territory, delivery cost and royalty option. Amazon’s 70% KDP formula, for example, is based on list price excluding VAT and delivery costs. Asking for your actual retailer royalty is more defensible than pretending one generic percentage covers every case.
Formula: books × usual average price × (1 − discount).
Why: this answers a pricing question before platform-specific rules: what does the bundle cost relative to buying the books separately?
Optional keep estimate: bundle price × the percentage rate you enter. Fixed fees, delivery costs, VAT and retailer price-band rules are deliberately not invented.
Formula: current monthly royalty = current royalty per sale × current sales. Required new sales = current monthly royalty ÷ new royalty per sale.
Rounding: exact decimal arithmetic is used before rounding required sales up, so an unchanged royalty cannot accidentally require an extra sale.
Why: this isolates the useful question: how much volume must change for the new per-sale economics to produce the same royalty total?
Formula: remaining words ÷ estimated writing days.
Calendar rule: dates are counted as calendar dates from today through the deadline, inclusive, using date-only arithmetic so daylight-saving changes cannot shift the answer.
Writing-day rule: calendar days × writing days per week ÷ 7, rounded down. The daily word target is rounded up.
Why: without knowing your exact weekdays, the schedule is an estimate. We round the proportional estimate down without inventing a minimum session. Fewer than one estimated writing day means no daily target is available; the actual weekday schedule may differ.