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Practical guide

Deposits, bonds, savings and ROI — what should be compared?

Each investment calculator answers a different question. A deposit covers one term, bonds may compound, savings add contributions, while ROI compares after-cost outlay and proceeds, with optional annualisation for two dates.

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Simple Tools editorial teamReviewed: 22 September 2026

Match the model to cash flows

A deposit paid at maturity may use simple interest. Multi-year reinvested capital requires compounding, while a savings plan also needs contribution dates.

Deposits use exact dates and tax at payout. The bond model covers named September 2026 issues: TOS/EDO compound, while COI pays coupons. Savings schedules distinguish start- and end-of-month contributions.

Gross, net and real value

Gross interest is not the amount available after tax and fees. A nominal value also omits loss of purchasing power from inflation.

When comparing products, use the same tax convention, time period and realistic contribution schedule.

ROI does not measure everything

Simple ROI divides gain by cost. Two projects with 20% ROI differ greatly if one takes a month and the other five years.

Annualisation assumes one initial outlay and one final receipt. It is not IRR/XIRR for intermediate cash flows and does not automatically account for tax or risk.

Frequently asked questions

What does this guide explain about “Match the model to cash flows”?

A deposit paid at maturity may use simple interest. Multi-year reinvested capital requires compounding, while a savings plan also needs contribution dates. Deposits use exact dates and tax at payout. The bond model covers named September 2026 issues: TOS/EDO compound, while COI pays coupons. Savings schedules distinguish start- and end-of-month contributions.

What does this guide explain about “Gross, net and real value”?

Gross interest is not the amount available after tax and fees. A nominal value also omits loss of purchasing power from inflation. When comparing products, use the same tax convention, time period and realistic contribution schedule.

What does this guide explain about “ROI does not measure everything”?

Simple ROI divides gain by cost. Two projects with 20% ROI differ greatly if one takes a month and the other five years. Annualisation assumes one initial outlay and one final receipt. It is not IRR/XIRR for intermediate cash flows and does not automatically account for tax or risk.

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