Dollar Cost Averaging vs Lump Sum Calculator
Compare a lump sum with equal monthly purchases across five fixed synthetic return paths. Review overlapping window results, strict wins, and ties without mistaking the scenarios for market history.
Earlier editorial review
- Reviewed
- 2026-09-18
- Methodology
- 1.1
Sources
How to use the comparison
Enter an amount available now, choose how many months to spread staged purchases across, and select one of five synthetic return profiles. The model compares a full investment made before the first monthly return with equal installments invested at the beginning of each month.
The selected profile has 240 months. For a purchase period of N months, the tool analyzes 240 minus N plus 1 overlapping windows. It reports average, highest, and lowest return for each method, strict win counts, ties, and counts grouped by the lump sum return in each window.
Change the purchase period while keeping the profile fixed to see how the modeled timing comparison changes. Read the number of windows with the win counts. Overlapping windows reuse the same synthetic path and should not be read as independent market observations.
The paths are fixed seeded simulations, not observations from named indexes or funds. The comparison does not predict future returns or choose an investment method for you. Actual outcomes depend on the assets, costs, cash return, taxes, and schedule that apply to an investor.
Step by step
- Enter the amount available before the first modeled monthly return.
- Choose the number of equal monthly purchases and one synthetic return profile.
- Review strict wins, ties, return ranges, and the number of overlapping windows.
Worked example
For a two month arithmetic example, start with 120 and use monthly returns of −10% followed by +10%. Lump sum ends at 118.80, a return of −1%. Two beginning of month installments of 60 end at 125.40, a return of +4.5%, so staged investing wins this one window.
The example has only one window and demonstrates timing arithmetic. It is not a historical observation or an estimate of how often either strategy will win.
When this comparison is useful
Use the comparison to inspect how investing timing affects results under a clearly specified synthetic return path when the full amount is already available.
Use Sequence of Returns to study the effect of return order, Historical Drawdown to review market declines, or Cash Drag Estimator to examine the cost of cash waiting outside an investment.
Common interpretation errors
Do not describe the scenario names as actual index data or treat a win percentage as a future probability. The windows overlap and come from one fixed synthetic path. A different return sequence can produce a different comparison.
Do not confuse investing an existing amount gradually with investing new income as it arrives. The calculator assumes the full amount is available at the start. If contributions arrive over time, that is a different cash flow pattern.
How the calculation works
Lump sum factor = ∏(1 + rⱼ) across N months. Staged factor = (1 ÷ N) × Σᵢ∏(1 + rⱼ) from purchase month i through month N. Window count = M − N + 1. A win requires one ending factor to be strictly greater than the other.
Each installment is invested at the beginning of its month. The tool reports percentage returns on the same starting amount. Windows overlap, ties are separate, and the five fixed paths are synthetic illustrations.
Assumptions and limits
The five return profiles are deterministic synthetic paths of 240 monthly returns. They are not actual index histories, fund records, or historical backtests. The calculator does not estimate future win probabilities from a distribution of market outcomes. Every result is limited to the selected path and period.
Key terms
- Overlapping window
- Every possible overlapping window of the selected length within one 240 month return path.
- Synthetic return path
- A reproducible sequence generated from fixed inputs for illustration, not recorded market or fund performance.
- Tie
- A window where both methods finish at exactly the same modeled value. A tie is not counted as a win for either method.
Compare related questions
Compare investing an available amount at once with spreading that same amount across monthly purchases. This tool uses fixed synthetic return paths to show how entry timing changes the modeled result.
Use Lump Sum vs Periodic to compare fixed return assumptions, Sequence of Returns to examine return order, Historical Drawdown to review past market declines, and Cash Drag Estimator to model the effect of cash held aside.
FAQ
How are the two methods compared?
The selected path contains 240 fixed synthetic monthly returns. For a period of N months, the calculator compares every overlapping N month window. Lump sum invests the full starting amount before the window begins. Staged investing divides it into N equal purchases at the beginning of each month. Cash waiting for a later purchase earns no interest in the model.
How are wins and ties counted?
A strict win means one method finishes with a higher modeled value than the other in that window. Exact ties are counted separately, so neither method receives a win for a tie. The win percentages use all windows as the denominator and apply only to the selected synthetic path and purchase period.
Are these actual market histories?
They are reproducible synthetic scenarios generated from fixed seeds. Their profile names describe broad return characteristics. They are not downloaded index history, actual fund returns, historical backtests, or forecasts. The tool does not estimate the future probability of either strategy winning.
When are staged purchases invested?
Each purchase is made at the beginning of its month and receives that month’s modeled return. The initial amount is split evenly across the selected number of months. The model assigns no interest to cash that has not yet been invested.
What changes when I extend the purchase period?
A longer period leaves more of the starting amount outside the modeled investment for longer. That can reduce exposure to a decline near the beginning, while lowering participation when returns rise. This comparison omits cash interest, so it does not measure the result of holding money in an interest bearing account.
Related tools
Lump Sum vs Periodic compares fixed return assumptions. Sequence of Returns focuses on return order, Historical Drawdown summarizes past market declines, and Cash Drag Estimator models cash held outside an investment.