Development of a mortgage calculator using 1C-Bitrix

Development of a Mortgage Calculator on 1C-Bitrix

A mortgage calculator is one of the most technically rich financial tools on a website. Unlike simple "area × price" multiplication, it involves annuity payment formulas, down payment accounting, insurance, and differentiated vs. annuity repayment types. At the same time, the user expects an instant answer — any delay is perceived as a site error.

The math: annuity payment

An annuity payment is the same amount every month. The formula:

P = S × r / (1 - (1 + r)^(-n))

Where:

• P — monthly payment
• S — loan amount (property price minus down payment)
• r — monthly interest rate (annual ÷ 12 ÷ 100)
• n — number of months

Implementation in PHP:

namespace MyProject\Services;

class MortgageCalculator
{
    public static function calculate(
        float $propertyPrice,
        float $downPayment,
        float $annualRate,
        int   $termYears
    ): array {
        $loanAmount  = $propertyPrice - $downPayment;
        $termMonths  = $termYears * 12;
        $monthlyRate = $annualRate / 12 / 100;

        if ($monthlyRate == 0) {
            // Zero-interest installment (rare but happens)
            $monthlyPayment = $loanAmount / $termMonths;
        } else {
            $monthlyPayment = $loanAmount
                * $monthlyRate
                * pow(1 + $monthlyRate, $termMonths)
                / (pow(1 + $monthlyRate, $termMonths) - 1);
        }

        $totalPayment   = $monthlyPayment * $termMonths;
        $totalInterest  = $totalPayment - $loanAmount;
        $overpaymentPct = ($totalInterest / $loanAmount) * 100;

        return [
            'loan_amount'        => round($loanAmount, 2),
            'monthly_payment'    => round($monthlyPayment, 2),
            'total_payment'      => round($totalPayment, 2),
            'total_interest'     => round($totalInterest, 2),
            'overpayment_pct'    => round($overpaymentPct, 1),
            'down_payment_pct'   => round(($downPayment / $propertyPrice) * 100, 1),
        ];
    }

    public static function buildPaymentSchedule(
        float $loanAmount,
        float $annualRate,
        int   $termMonths
    ): array {
        $schedule     = [];
        $monthlyRate  = $annualRate / 12 / 100;
        $monthlyPayment = $loanAmount
            * $monthlyRate
            * pow(1 + $monthlyRate, $termMonths)
            / (pow(1 + $monthlyRate, $termMonths) - 1);

        $balance = $loanAmount;

        for ($month = 1; $month <= $termMonths; $month++) {
            $interestPayment = $balance * $monthlyRate;
            $principalPayment = $monthlyPayment - $interestPayment;
            $balance -= $principalPayment;

            $schedule[] = [
                'month'     => $month,
                'payment'   => round($monthlyPayment, 2),
                'principal' => round($principalPayment, 2),
                'interest'  => round($interestPayment, 2),
                'balance'   => round(max($balance, 0), 2),
            ];
        }

        return $schedule;
    }
}

Payment schedule: table and chart

A full repayment schedule is a competitive advantage. The user can see how the principal debt decreases and how much goes toward interest in different periods. Chart.js or ApexCharts are used for visualization — lightweight libraries without unnecessary dependencies.

// Chart data comes from the server via AJAX
async function updateChart(params) {
    const response = await fetch('/ajax/mortgage/schedule/', {
        method: 'POST',
        body: new URLSearchParams({ ...params, sessid: BX.bitrix_sessid() }),
    });
    const data = await response.json();

    if (!data.success) return;

    // Group by year for the chart
    const years = [];
    const principalByYear = [];
    const interestByYear  = [];

    data.schedule.forEach((month, i) => {
        const yearIndex = Math.floor(i / 12);
        if (!years[yearIndex]) {
            years[yearIndex] = `Year ${yearIndex + 1}`;
            principalByYear[yearIndex] = 0;
            interestByYear[yearIndex]  = 0;
        }
        principalByYear[yearIndex] += month.principal;
        interestByYear[yearIndex]  += month.interest;
    });

    renderStackedBarChart(years, principalByYear, interestByYear);
}

Insurance accounting

Banks require insurance. The calculator must show the real cost:

• Property insurance — 0.1–0.5% of property value per year
• Life and health insurance — 0.3–1.5% of the loan balance (optional, but reduces the rate)
• Title insurance — relevant for the secondary market

$annualInsurance = $loanAmount * ($insuranceRate / 100);
$monthlyInsurance = $annualInsurance / 12;
$totalMonthlyPayment = $monthlyPayment + $monthlyInsurance;

Bank comparison

For aggregators and developer websites — a comparison table of offers from several banks. Data is stored in an HL block and updated by a manager:

Integration with the request form

The calculation result is passed to the request form as hidden fields — the manager can see what the client was considering:

document.getElementById('submit-btn').addEventListener('click', () => {
    document.getElementById('hidden-loan-amount').value  = currentResult.loan_amount;
    document.getElementById('hidden-monthly-pay').value  = currentResult.monthly_payment;
    document.getElementById('hidden-annual-rate').value  = currentParams.annualRate;
    document.getElementById('hidden-term-years').value   = currentParams.termYears;
});

Timelines

A mortgage calculator works when the math is correct and the UX is not intimidating. The client should get an answer within 2–3 seconds of interacting with the form — that is the criterion for a successful tool.