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Methodology

Business Interest Cost Planner methodology

Roll principal through two editable periods and compare interest cost using user-entered rates, days and day-count bases.
Duc Nguyen X.By Duc Nguyen X.· Founder, Margin101Last reviewed: View update historyMethodology

Educational only: Business decision support, not accounting, tax or legal advice.

Privacy: Calculations run locally; Margin101 does not receive your commercial inputs.

Update policy: Reviewed when formulas or official dependencies change.

Scope: Market-neutral small-business planning using your own assumptions.

1. Formulas and units

Closing principal for each period
closingPrincipal = openingPrincipal + draws − repayments

Where

openingPrincipal
First opening principal (currency units)Source: User assumption
draws
Draws in each period (currency units)Source: User assumption
repayments
Repayments in each period (currency units)Source: User assumption
closingPrincipal
Closing principal for a period (currency units)Source: Calculated output
Average principal for each period
averagePrincipal = (openingPrincipal + closingPrincipal) ÷ 2, assuming draws and repayments occur evenly through the period

Where

openingPrincipal
First opening principal (currency units)Source: User assumption
draws
Draws in each period (currency units)Source: User assumption
repayments
Repayments in each period (currency units)Source: User assumption
averagePrincipal
Average principal for a period (currency units)Source: Calculated output
closingPrincipal
Closing principal for a period (currency units)Source: Calculated output
Interest cost for each period
interestCost = averagePrincipal × annualInterestRate × days ÷ dayCountBasis

Where

annualInterestRate
Annual interest rate in each period (decimal rate)Source: User assumption
days
Days in each period (days)Source: User assumption
dayCountBasis
Day-count basis in each period (days per year)Source: User assumption
averagePrincipal
Average principal for a period (currency units)Source: Calculated output
interestCost
Interest cost for a period (currency units)Source: Calculated output
totalInterestCost
Collection total interest (currency units)Source: Calculated output
Collection total interest
totalInterestCost = sum of interestCost across all chronological periods

Where

periods
Chronological interest periods (collection of 1 to 60 rows)Source: User assumption
interestCost
Interest cost for a period (currency units)Source: Calculated output
totalInterestCost
Collection total interest (currency units)Source: Calculated output
Ending principal
endingPrincipal = closingPrincipal from the final period

Where

closingPrincipal
Closing principal for a period (currency units)Source: Calculated output
endingPrincipal
Ending principal (currency units)Source: Calculated output

Money inputs and outputs use the currency selected in the scenario without changing the canonical methodology. Rates, margins, utilisation and buffers are entered as percentages and converted to decimal values for calculation.

2. Worked example

Input assumptions

The worked rows below come directly from the registered engine and visible default fixture.

Calculation and outputs

Example

The worked rows below come directly from the registered engine and visible default fixture.

period-1 average principal
10,500 currency units
period-1 interest cost
1,050 currency units
period-1 closing principal
11,000 currency units
period-2 average principal
10,500 currency units
period-2 interest cost
1,050 currency units
period-2 closing principal
10,000 currency units
Collection total interest
2,100 currency units
Ending principal
10,000 currency units

The exact engine-derived outputs are shown in the labelled rows below.

Interpretation

Interest follows the entered average principal, annual rate and contractual day-count basis.

3. Validation and boundary checks

  • The collection must contain between 1 and 60 uniquely identified periods.
  • Draws, repayments, rates, days and day-count bases must remain within the visible boundaries.
  • Repayments cannot exceed opening principal plus draws for a period.
  • Opening principal rolls forward exactly from the prior closing principal.

4. Assumptions and source classification

  • All balances, rates, timing and coverage thresholds are user-supplied records or explicit assumptions.
  • No lender covenant, market rate, policy threshold or future cash flow is inferred.
  • Each period opening principal must equal the preceding period closing principal.
  • The midpoint balance assumes draws and repayments occur evenly through each period; use shorter rows when timing is materially uneven.

This planner has no current policy-data dependency. Its commercial assumptions are user supplied. Registered family-level regression suites exercise the shared business-logic engine and worked-result reconciliation.

5. Limitations

  • The model excludes lender fees, taxes, covenant definitions and lender-specific credit assessments.
  • It does not replace accounting, tax, legal or financial advice.

This is educational decision support, not tax, accounting, legal or financial advice. Check the treatment of your actual transactions under the rules that apply to your business and seek qualified advice where appropriate.

6. Update and evidence policy

Registered family-level suites test formula invariants and example reconciliation; the release ledger records that coverage without claiming a separate oracle for every line of public copy. There is no official threshold or benchmark to refresh for this planner. Commercial inputs remain user-supplied because they vary by business and contract.

Change history

  1. : Initial public release of the Business Interest Cost planner and methodology.

Related reading

Guides to interpret the decision and its assumptions.

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