Stamp duty land tax calculator that allows variable bands.

sdlt.php

<?php

$bands = [
    [
        'threshold' => 0,
        'primaryRate' => 0,
        'secondaryRate' => 0.03,
    ],
    [
        'threshold' => 125000,
        'primaryRate' => 0.02,
        'secondaryRate' => 0.05,
    ],
    [
        'threshold' => 250000,
        'primaryRate' => 0.05,
        'secondaryRate' => 0.08,
    ],
    [
        'threshold' => 925000,
        'primaryRate' => 0.1,
        'secondaryRate' => 0.13,
    ],
    [
        'threshold' => 1500000,
        'primaryRate' => 0.12,
        'secondaryRate' => 0.15,
    ],
];

$price = 960000;
$isSecondary = true;

$totalTax = 0;

foreach ($bands as $index => $band) {
    $rate = $isSecondary ? $band['secondaryRate'] : $band['primaryRate'];
    $nextThreshold = 0;

    if (isset($bands[$index + 1])) {
        $nextThreshold = $bands[$index + 1]['threshold'];
    }

    $taxableSum = max(0, min($price - $band['threshold'], $nextThreshold - $band['threshold']));

    $totalTax += $taxableSum * $rate;
}

echo "{$totalTax}\n";

stamp-duty-calculator.js

function calculateTaxForBands(amount, bands) {
  var totalTax = 0;

  for (var i = 0; i < bands.length; i++) {
    var nextFrom = i === bands.length - 1 ? amount : bands[i + 1].from;
    totalTax += Math.max(0, Math.min(amount - bands[i].from, nextFrom - bands[i].from)) * bands[i].rate;
  }

  return totalTax;
}

uk-sdlt-bands.js

var bands = {
  uk: {
    mainHome: [
      { from: 0, rate: 0 },
      { from: 125000, rate: 0.02 },
      { from: 250000, rate: 0.05 },
      { from: 925000, rate: 0.1 },
      { from: 1500000, rate: 0.12 }
    ],
    secondHome: [
      { from: 0, rate: 0.03 },
      { from: 125000, rate: 0.05 },
      { from: 250000, rate: 0.08 },
      { from: 925000, rate: 0.13 },
      { from: 1500000, rate: 0.15 }
    ]
  }
};

usage.js

calculateTaxForBands(80000, bands.uk.mainHome); // 0
calculateTaxForBands(80000, bands.uk.secondHome); // 2400
calculateTaxForBands(175000, bands.uk.mainHome); // 1000
calculateTaxForBands(325000, bands.uk.mainHome); // 6250
calculateTaxForBands(1200000, bands.uk.mainHome); // 63750
calculateTaxForBands(4100000, bands.uk.mainHome); // 405750