loanInstallmentAmount()
Calculate one loan installment for equal (annuity) or decreasing schedules — the building block for mortgage, car-loan, and equipment-financing calculators.
Syntax
Every formula in Calcopolis must start with = in the Value field:
= loanInstallmentAmount(amount, interestRate, months, installmentNo, commission, installments)
| Part | Rule |
|---|---|
| Function name | loanInstallmentAmount — camelCase |
amount |
Principal loan amount |
interestRate |
Annual rate as decimal (0.05 = 5% per year) |
months |
Loan term in months |
| Optional args | installmentNo (default 1), commission (decimal fee on principal), installments ("EQUAL" or "DECREASING") |
| Return value | Payment amount for the requested installment number |
Valid examples
= loanInstallmentAmount(loan_amount, interest_rate/100, term_months)
= loanInstallmentAmount(loan_amount, interest_rate/100, term_months, 1, 0, "EQUAL")
= roundTo(loanInstallmentAmount(amount, rate/100, months), 2)
Invalid / unsupported
| Don't write | Why |
|---|---|
interest_rate without /100 |
Convert visitor percent input: interest_rate/100 when they type 5 for 5% |
loanInstallmentAmount(amount, 5, 360) |
Rate must be decimal 0.05, not 5 |
installments: EQUAL unquoted |
Pass "EQUAL" or "DECREASING" as a quoted string |
Parameters
| Argument | Description |
|---|---|
amount |
Loan principal. |
interestRate |
Yearly interest as decimal. Divide form percent by 100. |
months |
Total number of monthly payments. |
installmentNo |
Which payment to compute (1 = first month). Default 1. |
commission |
One-time fee as decimal of principal (0.01 = 1%). Default 0. |
installments |
"EQUAL" (fixed payment) or "DECREASING" (declining principal). Default "EQUAL". |
When to use loanInstallmentAmount()
Use for monthly payment outputs on finance calculators:
| Situation | What loanInstallmentAmount() does |
|---|---|
| Mortgage quote | Principal + annual rate + term in years × 12 |
| Car loan | Optional commission folded into payment |
| First vs. last payment | Change installmentNo for amortization tables |
Combine with [loanTotalCost()](/docs/reference/formulas/loan-total-cost-function/) and [loanTotalInterest()](/docs/reference/formulas/loan-total-interest-function/) for full summary lines.
Practical examples (ecommerce & leads)
1. Standard mortgage payment
Inputs: Number loan_amount, interest_rate (percent), term_years.
| Variable | Formula |
|---|---|
term_months |
= term_years * 12 |
monthly_payment |
= roundTo(loanInstallmentAmount(loan_amount, interest_rate/100, term_months), 2) |
2. Loan with origination fee
commission = 0.02 for 2% fee on principal.
| Variable | Formula |
|---|---|
monthly_payment |
= roundTo(loanInstallmentAmount(loan_amount, interest_rate/100, term_months, 1, 0.02, "EQUAL"), 2) |
3. Decreasing installment schedule
| Variable | Formula |
|---|---|
first_payment |
= loanInstallmentAmount(loan_amount, interest_rate/100, term_months, 1, 0, "DECREASING") |
last_payment |
= loanInstallmentAmount(loan_amount, interest_rate/100, term_months, term_months, 0, "DECREASING") |
Where to use it
| Location | Supported |
|---|---|
| Result widget Value | Yes — = ... formulas |
| Standalone variable (Variables tab) | Yes |
| Number / other input default Value | Yes, if formula-driven |
| Visibility formula | Yes |
| Custom Text / Table cells | No — use {{ }} templates there, not = functions |
After editing, run Calculate (or enable Run calculation on load in project Options) to refresh results.
Related
- [
roundTo()](/docs/reference/formulas/round-to-function/) - [
loanRRSO()](/docs/reference/formulas/loan-rrso-function/) - [
loanTotalInterest()](/docs/reference/formulas/loan-total-interest-function/) - [
loanTotalCost()](/docs/reference/formulas/loan-total-cost-function/) - Understanding Formulas and Variables —
=syntax and variables - Building Your First Calculator — end-to-end quote calculator