STP Calculator

Created by Arunangshu DasReviewed Nov 10, 2026

Model fixed monthly transfers between a source and destination investment.

Compare both timings and see how the two balances and funded transfers reconcile.

Combined ending value

Enter a transfer plan or load the worked example.

Month numbers start at 1 for both timings. Each funded transfer is deducted from the source and credited to the destination immediately. Growth uses equal model months and a constant assumed return.

Understand what drives the projection

Walk through transfer order, partial funding and separate source and destination returns.

Read the complete STP guide

Formula and methodology

Combined ending value = opening source balance + signed modeled growth.

Convert each effective annual decimal rate A to its monthly factor (1 + A)^(1/12). Beginning timing moves the funded amount before both balances grow. End timing grows both balances first. Each funded amount is the lesser of the request and available source.

Definitions

A systematic transfer plan (STP) moves amounts from a source investment to a destination investment. This model starts with one source balance and an empty destination.

Verified worked examples

With a source balance of 1,000, a monthly request of 400, zero returns and three months, transfers are 400, 400 and 200. The source ends at zero, the destination at 1,000 and the combined value at 1,000. Unmet requests total 200 and signed growth is zero.

Interpretation

The combined ending value equals opening source plus signed modeled growth. Requested less funded equals unmet. A full transfer can deplete the source before any request goes unfunded.

Inputs and interpretation

Enter the opening source, fixed monthly request, both effective annual percentages and complete months. Choose beginning or end timing. Read the balances, monthly rates and transfer outcomes together.

Assumptions and limitations

Both balances use equal model months and the destination starts at zero. The same funded amount leaves the source and enters the destination immediately. The first request is labeled month 1 for either timing.

Supported domain and common mistakes

Opening amount and monthly request range from 0 to 10^12, horizon from 0 to 1,200 months and each annual percentage from above −100% to 100%. Decimals allow 12 fractional places. Arithmetic uses at least 100 significant digits, flags unresolved funding thresholds and rejects combined ending values above 10^30.

Do not add funded transfers to the combined value again. Check whether another calculator applies returns before or after the switch, and whether the annual rates use the same basis.

Frequently asked questions

How does an STP calculator work?

It moves a fixed requested amount from one modeled investment to another each month, subject to available source funds. Each balance has its own assumed rate.

What happens if the source runs short?

The model transfers the available amount and reports the remainder as unmet. The first shortfall shows the month and amount actually funded.

Do transfers count as gains?

No. The source debit and destination credit cancel in combined value. Gains or losses come from the growth applied to each balance.

What happens if both rates are equal?

The combined ending value is the same for all transfer schedules and either timing under equal rates and no fees. The distribution between the balances can still differ.

What happens at zero months?

The source retains its opening balance, the destination stays at zero and no requests are made.

Can the rates be negative?

Yes, each annual rate can be negative as long as it is above −100%. A negative rate reduces the balance exposed to it.

Sources and educational scope

Educational constant-return projection in one currency. It excludes dates, eligibility, settlement, NAV, taxes, fees, inflation, FX and changing market returns. It does not provide personalized investment advice.

Reviewed 2026-11-10. AI-assisted content under operator editorial responsibility. No independent expert financial review is claimed.