Guide
Start vs End of Month Contributions (Does Timing Matter?)
Timing changes how long each deposit compounds. Here’s a simple start-of-month vs end-of-month example and how to test both.
Published: March 12, 2025 · Updated: December 21, 2025 · By FinToolSuite Editorial
Try the calculator
Run start vs end of month scenarios with the same inputs to see the difference.
Open the calculatorQuick answer
Start-of-month deposits compound slightly longer than end-of-month deposits. The difference is usually modest but grows with time, rate, and contribution size. More on monthly deposits: monthly contributions and compounding.
Disclaimer
Educational purposes only; not financial advice. Examples are illustrative; real returns vary and investments can go down as well as up. Fees, taxes, inflation, and rules vary by provider and country.
What “start vs end of month” means
Start: deposit at the beginning of the period, giving it more time to earn interest. End: deposit after the period’s growth, which is a more conservative assumption. Some calculators ask this because timing changes how long contributions compound.
Example: start vs end
Starting amount: £0. Monthly contribution: £100. Rate: 5% (illustrative). Time: 10 years. Compounding: monthly.
| Timing | Ending balance (approx.) |
|---|---|
| Start of month | ~£15,600 |
| End of month | ~£15,300 |
| Difference | ~£300 |
Illustration only—the gap grows with higher contributions, higher rates, or longer timelines.
When timing matters most
- Longer time horizons
- Higher rates
- Larger monthly contributions
- More frequent deposits
How to model timing in the calculator
In the calculator, “start of period” means deposits are added before interest; “end of period” means deposits are added after interest. Run both options as two scenarios to see the range.
Open the calculatorCommon mistakes
- Assuming timing never matters.
- Changing multiple inputs when comparing timing.
- Assuming start-of-month always matches real deposit dates.
FAQ
What does “contribute at start of period” mean?
Deposits are added before interest is calculated for that period.
Is end-of-month more conservative?
Yes. Deposits compound for slightly less time, which is a more cautious assumption.
How big is the difference in practice?
Often modest, but it grows with higher contributions, higher rates, or longer horizons.
Does timing matter for weekly contributions too?
Yes, though differences per period are smaller; frequency and timing both affect compounding.
What if my deposit date varies?
Pick the assumption closest to your usual pattern, or test both start and end to bracket results.
How do I model salary-day deposits?
If paid near month start, the start-of-month assumption is closer; if later, end-of-month may be more realistic.
How do fees/taxes/inflation affect this?
They reduce effective outcomes; timing still matters but overall results shift lower.
Which option should I pick in a calculator?
Use the option that matches your deposit pattern, or test both to see the range.
Compare timing in the tool
Save a start-of-month scenario and an end-of-month scenario to see how much timing shifts the totals.
Open the calculator