Development of an installment calculator for 1C-Bitrix

Development of an Installment Calculator on 1C-Bitrix

Installment is a marketing tool, not a financial product. The key difference from a loan: there are no interest charges for the buyer (they are either absent or compensated by the seller to the bank). An installment calculator is simpler mathematically, but more complex logically: it needs to reflect different installment periods, early repayment conditions, and possible processing fees.

Installment mechanics

In a classic zero-overpayment installment plan:

Monthly payment = Item price ÷ Number of months

The seller compensates the bank for the difference out of their margin or through a special agreement with a partner bank.

In a "0-0-24" installment (0% down payment, 0% rate, 24 months) — hidden conditions often include mandatory insurance or an account maintenance fee. The calculator must show the real total amount.

PHP implementation of the calculation

namespace MyProject\Services;

class InstallmentCalculator
{
    /**
     * @param float  $amount      Item/service price
     * @param int    $months      Installment term
     * @param float  $downPayment Down payment (0 if none)
     * @param float  $bankRate    Actual bank rate (compensated by seller)
     * @param float  $commission  One-time commission (0 if none)
     */
    public static function calculate(
        float $amount,
        int   $months,
        float $downPayment = 0,
        float $bankRate    = 0,
        float $commission  = 0
    ): array {
        $loanAmount      = $amount - $downPayment;
        $commissionFixed = $amount * ($commission / 100);

        if ($bankRate == 0) {
            // Classic zero-interest installment
            $monthlyPayment = $loanAmount / $months;
            $totalPayment   = $loanAmount + $commissionFixed;
            $overpayment    = $commissionFixed;
        } else {
            // Installment with an actual rate (paid by seller, not buyer)
            $monthlyRate    = $bankRate / 12 / 100;
            $monthlyPayment = $loanAmount * $monthlyRate
                * pow(1 + $monthlyRate, $months)
                / (pow(1 + $monthlyRate, $months) - 1);
            $totalPayment   = $monthlyPayment * $months + $commissionFixed;
            $overpayment    = $totalPayment - $loanAmount;
        }

        return [
            'down_payment'    => round($downPayment, 2),
            'loan_amount'     => round($loanAmount, 2),
            'monthly_payment' => round($monthlyPayment, 2),
            'commission'      => round($commissionFixed, 2),
            'total_payment'   => round($totalPayment, 2),
            'overpayment'     => round($overpayment, 2),
            'months'          => $months,
            'is_zero_percent' => ($overpayment == 0 || $overpayment == $commissionFixed),
        ];
    }
}

Configuration of installment options

Stores work with multiple partner banks, each offering different conditions. Storage in an HL block InstallmentPrograms:

When an amount is selected, the calculator automatically displays available installment programs.

UX: displaying options

The interface shows a comparison table of options:

function renderInstallmentTable(programs, amount) {
    const rows = programs
        .filter(p => amount >= p.min_amount && amount <= p.max_amount)
        .map(p => {
            const calc = calcInstallment(amount, p.months, p.down_pct, p.commission_pct);
            return `
                <tr>
                    <td>${p.bank_name}</td>
                    <td>${p.months} mo.</td>
                    <td>${formatMoney(calc.down_payment)} (${p.down_pct}%)</td>
                    <td>${formatMoney(calc.monthly_payment)}/mo.</td>
                    <td>${calc.overpayment > 0 ? formatMoney(calc.overpayment) : '0'}</td>
                    <td><button onclick="selectProgram(${p.id})">Select</button></td>
                </tr>
            `;
        });

    document.getElementById('installment-table').innerHTML = rows.join('');
}

Integration with the Bitrix cart

For online stores the installment calculator is embedded in the product page and cart. The selected installment option is saved in the order properties:

// Save the selected installment program to an order property
$order = \Bitrix\Sale\Order::load($orderId);
$propertyCollection = $order->getPropertyCollection();

$installmentProp = $propertyCollection->getItemByOrderPropertyCode('INSTALLMENT_PROGRAM');
if ($installmentProp) {
    $installmentProp->setValue(json_encode([
        'bank'    => $bankName,
        'months'  => $months,
        'payment' => $monthlyPayment,
    ]));
}

$order->save();

Installment widget on the product card

The minimum payment is displayed directly on the product page — standard e-commerce practice:

// In the product template
$minMonthlyPayment = InstallmentCalculator::getMinMonthlyPayment(
    $arResult['ITEM_PRICES'][0]['PRICE'],
    $availablePrograms
);

if ($minMonthlyPayment) {
    echo "from {$minMonthlyPayment} /mo. on installment";
}

Timelines

An installment calculator lowers the psychological barrier for large purchases. Instead of "120,000 at once" the client sees "5,000 per month" — and those are very different decisions.