diff --git a/README.md b/README.md
index dc0a3ec..0f5c983 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,64 @@
 # uk-modulus-checking
-UK modulus checking.
+Modulus checking allows payment originators to confirm that customer codes and account numbers are compatible before submitting a Bacs Direct Credit of Direct Debit.
+
+## Status
+[![npm version][npm-image]][npm-url] [![build status][travis-image]][travis-url]
+
+## Installation
+Install the package via `npm`:
+
+```sh
+npm install uk-modulus-checking --save
+```
+
+## Usage
+
+### `new UkModulusChecking({ accountNumber, sortCode }).isValid()`
+
+This method validates if the given accountNumber and sortCode represent a valid `Faster Payment Account`.
+
+#### Arguments
+
+1. `accountNumber` *(string)*: The account number to validate.
+2. `sortCode` *(string)*: The sort code to validate.
+
+#### Returns
+*(boolean)*:  Returns `true` if the account is valid.
+
+#### Example
+```js
+new UkModulusChecking({ accountNumber: '15764273', sortCode: '938063' }).isValid();
+// => false
+
+new UkModulusChecking({ accountNumber: '66374958', sortCode: '089999' }).isValid();
+// => true
+
+new UkModulusChecking({ accountNumber: '66374958', sortCode: '08-99-99' }).isValid();
+// => true
+
+new UkModulusChecking({ accountNumber: '66374958', sortCode: '08-9999' }).isValid();
+// => true
+```
+
+## Tests
+
+```sh
+npm test
+```
+
+## Release
+
+```sh
+npm version [<newversion> | major | minor | patch] -m "Release %s"
+```
+
+## License
+MIT
+
+## Credits
+Many thanks to [bazerk/uk-modulus-checking](https://github.com/bazerk/uk-modulus-checking) for the original inspiration.
+
+[npm-image]: https://img.shields.io/npm/v/uk-modulus-checking.svg?style=flat-square
+[npm-url]: https://npmjs.org/package/uk-modulus-checking
+[travis-image]: https://img.shields.io/travis/uphold/uk-modulus-checking.svg?style=flat-square
+[travis-url]: https://img.shields.io/travis/uphold/uk-modulus-checking.svg?style=flat-square
diff --git a/package.json b/package.json
index 2498779..deedfe0 100644
--- a/package.json
+++ b/package.json
@@ -1,7 +1,7 @@
 {
   "name": "uk-modulus-checking",
   "version": "0.0.1",
-  "description": "Validating account numbers",
+  "description": "Validate a UK bank account number against a sort code using the VocaLink modulus check",
   "author": "Uphold",
   "license": "MIT",
   "homepage": "https://github.com/uphold/uk-modulus-checking",
diff --git a/src/constants.js b/src/constants.js
new file mode 100644
index 0000000..f05556d
--- /dev/null
+++ b/src/constants.js
@@ -0,0 +1,19 @@
+
+/* jscs:disable validateOrderInObjectKeys */
+export const positions = {
+  u: 0,
+  v: 1,
+  w: 2,
+  x: 3,
+  y: 4,
+  z: 5,
+  a: 6,
+  b: 7,
+  c: 8,
+  d: 9,
+  e: 10,
+  f: 11,
+  g: 12,
+  h: 13
+};
+/* jscs:enable validateOrderInObjectKeys */
diff --git a/src/index.js b/src/index.js
new file mode 100644
index 0000000..5a097ba
--- /dev/null
+++ b/src/index.js
@@ -0,0 +1,315 @@
+
+/**
+ * Module dependencies.
+ */
+
+import { positions } from './constants';
+import fs from 'fs';
+
+/**
+ * Export UkModulusChecking.
+ */
+
+export default class UkModulusChecking {
+
+  /**
+   * Constructor.
+   */
+
+  constructor({ accountNumber = '', sortCode = '' }) {
+    this.accountNumber = this.sanitize(accountNumber);
+    this.sortCode = this.sanitize(sortCode);
+    this.sortCodeSubstitutes = this.loadScsubtab();
+    this.weightTable = this.loadValacdos();
+  }
+
+  /**
+   * Get check weight.
+   */
+
+  getCheckWeight(check, number) {
+    if (check.exception === 2) {
+      if (this.pickPosition(number, 'a') !== 0 && this.pickPosition(number, 'g') !== 9) {
+        return [0, 0, 1, 2, 5, 3, 6, 4, 8, 7, 10, 9, 3, 1];
+      }
+
+      if (this.pickPosition(number, 'a') !== 0 && this.pickPosition(number, 'g') === 9) {
+        return [0, 0, 0, 0, 0, 0, 0, 0, 8, 7, 10, 9, 3, 1];
+      }
+    }
+
+    if (check.exception === 7) {
+      if (this.pickPosition(number, 'g') === 9) {
+        return [0, 0, 0, 0, 0, 0, 0, 0, check.c, check.d, check.e, check.f, check.g, check.h];
+      }
+    }
+
+    if (check.exception === 10) {
+      const ab = number.charAt(positions.a) + number.charAt(positions.b);
+
+      if (ab === '09' || ab === '99' && this.pickPosition(number, 'b') === 9) {
+        return [0, 0, 0, 0, 0, 0, 0, 0, check.c, check.d, check.e, check.f, check.g, check.h];
+      }
+    }
+
+    return [check.u, check.v, check.w, check.x, check.y, check.z, check.a, check.b, check.c, check.d, check.e, check.f, check.g, check.h];
+  }
+
+  /**
+   * Get number to be used in validation process. (sorting code + account number).
+   */
+
+  getNumber(check, number) {
+    let sortCode = this.sortCode;
+
+    number = number || this.accountNumber;
+
+    if (check.exception === 5) {
+      sortCode = this.getSubstitute(sortCode) || sortCode;
+    } else if (check.exception === 8) {
+      sortCode = '090126';
+    } else if (check.exception === 9) {
+      sortCode = '309634';
+    }
+
+    return `${sortCode}${number}`;
+  }
+
+  /**
+   * Get sorting code checks.
+   */
+
+  getSortCodeChecks() {
+    const checks = [];
+    const sortCode = parseInt(this.sortCode, 10);
+
+    for (const check of this.weightTable) {
+      // All checks containing the sort code in the `weight range` can/must be performed.
+      if (sortCode >= check.start && sortCode <= check.end) {
+        checks.push(check);
+      }
+
+      // There may be one or two entries in the table for the sorting code,
+      // depending on whether one or two modulus checks must be carried out.
+      if (checks.length === 2) {
+        return checks;
+      }
+    }
+
+    return checks;
+  }
+
+  /**
+   * Sorting code substitution.
+   */
+
+  getSubstitute(sortCode) {
+    for (const substitute of this.sortCodeSubstitutes) {
+      if (substitute.original === parseInt(sortCode, 10)) {
+        return parseInt(substitute.substitute, 10);
+      }
+    }
+
+    return parseInt(sortCode, 10);
+  }
+
+  /**
+   * Is check skippable.
+   */
+
+  isCheckSkippable(check, number) {
+    if (check.exception === 3 && (this.pickPosition(number, 'c') === 6 || this.pickPosition(number, 'c') === 9)) {
+      return true;
+    }
+
+    if (check.exception === 6 && this.pickPosition(number, 'a') >= 4 && this.pickPosition(number, 'a') <= 8 && this.pickPosition(number, 'g') === this.pickPosition(number, 'h')) {
+      return true;
+    }
+
+    return false;
+  }
+
+  /**
+   * Is check valid.
+   */
+
+  isCheckValid(check, number) {
+    number = this.getNumber(check, number);
+
+    if (this.isCheckSkippable(check, number)) {
+      return true;
+    }
+
+    const module = check.mod === 'MOD11' ? 11 : 10;
+    const weight = this.getCheckWeight(check, number);
+
+    // Multiply each number in the sorting code and account number with the corresponding number in the weight.
+    let weightedAccount = [];
+
+    for (let i = 0; i < 14; i++) {
+      weightedAccount[i] = parseInt(number.charAt(i), 10) * parseInt(weight[i], 10);
+    }
+
+    // Add all the results together.
+    if (check.mod === 'DBLAL') {
+      weightedAccount = weightedAccount.join('').split('');
+    }
+
+    let total = weightedAccount.reduce((previous, current) => parseInt(previous, 10) + parseInt(current, 10));
+
+    // This effectively places a financial institution number (580149) before the sorting code and account
+    // number which is subject to the alternate doubling as well.
+    if (check.exception === 1) {
+      total += 27;
+    }
+
+    // Calculate remainder.
+    const remainder = total % module;
+
+    // Exception handling.
+    if (check.exception === 4) {
+      return remainder === this.pickPosition(number, 'g') + this.pickPosition(number, 'h');
+    }
+
+    if (check.exception === 5) {
+      if (check.mod === 'DBLAL') {
+        if (remainder === 0 && this.pickPosition(number, 'h') === 0) {
+          return true;
+        }
+
+        return this.pickPosition(number, 'h') === 10 - remainder;
+      }
+
+      if (remainder === 1) {
+        return false;
+      }
+
+      if (remainder === 0 && this.pickPosition(number, 'g') === 0) {
+        return true;
+      }
+
+      return this.pickPosition(number, 'g') === 11 - remainder;
+    }
+
+    return remainder === 0;
+  }
+
+  /**
+   * Is valid.
+   */
+
+   isValid() {
+     const checks = this.getSortCodeChecks();
+
+     // If no range is found that contains the sorting code, there is no modulus check that can be performed.
+     // The sorting code and account number should be presumed valid unless other evidence implies otherwise.
+     if (checks.length === 0) {
+       return true;
+     }
+
+     const firstCheck = checks[0];
+
+     if (this.isCheckValid(firstCheck)) {
+       if (checks.length === 1 || [2, 9, 10, 11, 12, 13, 14].indexOf(firstCheck.exception) !== -1) {
+         return true;
+       }
+
+       // Verify second check.
+       return this.isCheckValid(checks[1]);
+     }
+
+     if (firstCheck.exception === 14) {
+       if ([0, 1, 9].indexOf(parseInt(this.accountNumber.charAt(7), 10)) === -1) {
+         return false;
+       }
+
+       //  If the 8th digit is 0, 1 or 9, then remove the digit from the account number and insert a 0 as the 1st digit for check purposes only
+       return this.isCheckValid(checks[0], `0${this.accountNumber.substring(7, 0)}`);
+     }
+
+     if (checks.length === 1 || [2, 9, 10, 11, 12, 13, 14].indexOf(firstCheck.exception) === -1) {
+       return false;
+     }
+
+     // Verify second check.
+     return this.isCheckValid(checks[1]);
+   }
+
+  /**
+   * Load scsubtab file.
+   */
+
+  loadScsubtab() {
+    const content = fs.readFileSync(`${__dirname}/data/scsubtab.txt`, 'utf8');
+    const scsubtab = [];
+
+    content.split('\r\n').forEach((line) => {
+      const data = line.split(/\s+/);
+
+      scsubtab.push({
+        original: parseInt(data[0], 10),
+        substitute: parseInt(data[1], 10)
+      });
+    });
+
+    return scsubtab;
+  }
+
+  /**
+   * Load valacdos file.
+   */
+
+  loadValacdos() {
+    const content = fs.readFileSync(`${__dirname}/data/valacdos-v370.txt`, 'utf8');
+    const valacdos = [];
+
+    content.split('\r\n').forEach((line) => {
+      const data = line.split(/\s+/);
+
+      /* jscs:disable validateOrderInObjectKeys */
+      valacdos.push({
+        start: parseInt(data[0], 10),
+        end: parseInt(data[1], 10),
+        mod: data[2],
+        u: parseInt(data[3], 10),
+        v: parseInt(data[4], 10),
+        w: parseInt(data[5], 10),
+        x: parseInt(data[6], 10),
+        y: parseInt(data[7], 10),
+        z: parseInt(data[8], 10),
+        a: parseInt(data[9], 10),
+        b: parseInt(data[10], 10),
+        c: parseInt(data[11], 10),
+        d: parseInt(data[12], 10),
+        e: parseInt(data[13], 10),
+        f: parseInt(data[14], 10),
+        g: parseInt(data[15], 10),
+        h: parseInt(data[16], 10),
+        exception: parseInt(data[17], 10) || null
+      });
+      /* jscs:enable validateOrderInObjectKeys */
+    });
+
+    return valacdos;
+  }
+
+  /**
+   * Pick position in number.
+   */
+
+   pickPosition(number, position) {
+     return parseInt(number.charAt(positions[position]), 10);
+   }
+
+  /**
+   * Sanitize.
+   */
+
+  sanitize(value) {
+    if (typeof value === 'string' || value instanceof String) {
+      return value.replace(/-/g, '');
+    }
+
+    throw new Error('Invalid value');
+  }
+}
diff --git a/test/index_test.js b/test/index_test.js
new file mode 100644
index 0000000..773c93c
--- /dev/null
+++ b/test/index_test.js
@@ -0,0 +1,67 @@
+
+/**
+ * Module dependencies.
+ */
+
+import UkModulusChecking from './../src';
+
+const accounts = {
+  invalid: [
+    { accountNumber: '15763217', sortCode: '938063' },
+    { accountNumber: '15764264', sortCode: '938063' },
+    { accountNumber: '15764273', sortCode: '938063' },
+    { accountNumber: '58716970', sortCode: '203099' },
+    { accountNumber: '64371388', sortCode: '118765' },
+    { accountNumber: '66374959', sortCode: '089999' },
+    { accountNumber: '66831036', sortCode: '203099' },
+    { accountNumber: '88837493', sortCode: '107999' }
+  ],
+  valid: [
+    { accountNumber: '00000190', sortCode: '180002' },
+    { accountNumber: '02355688', sortCode: '309070' },
+    { accountNumber: '06774744', sortCode: '086090' },
+    { accountNumber: '07806039', sortCode: '938611' },
+    { accountNumber: '09123496', sortCode: '871427' },
+    { accountNumber: '11104102', sortCode: '074456' },
+    { accountNumber: '12345112', sortCode: '074456' },
+    { accountNumber: '12345668', sortCode: '309070' },
+    { accountNumber: '12345677', sortCode: '309070' },
+    { accountNumber: '28748352', sortCode: '827101' },
+    { accountNumber: '34012583', sortCode: '070116' },
+    { accountNumber: '41011166', sortCode: '200915' },
+    { accountNumber: '42368003', sortCode: '938600' },
+    { accountNumber: '46238510', sortCode: '871427' },
+    { accountNumber: '46238510', sortCode: '872427' },
+    { accountNumber: '55065200', sortCode: '938063' },
+    { accountNumber: '63748472', sortCode: '202959' },
+    { accountNumber: '63849203', sortCode: '134020' },
+    { accountNumber: '64371389', sortCode: '118765' },
+    { accountNumber: '66374958', sortCode: '089999' },
+    { accountNumber: '73688637', sortCode: '820000' },
+    { accountNumber: '73988638', sortCode: '827999' },
+    { accountNumber: '88837491', sortCode: '107999' },
+    { accountNumber: '99123496', sortCode: '871427' },
+    { accountNumber: '99345694', sortCode: '309070' },
+    { accountNumber: '99345694', sortCode: '772798' }
+  ]
+};
+
+/**
+ * Test `UkModulusChecking`.
+ */
+
+describe('UkModulusChecking', () => {
+  describe('isValid()', () => {
+    accounts.invalid.forEach((account) => {
+      it(`should return false if sort code is ${account.sortCode} and account number is ${account.accountNumber}`, () => {
+        new UkModulusChecking({ accountNumber: account.accountNumber, sortCode: account.sortCode }).isValid().should.be.false();
+      });
+    });
+
+    accounts.valid.forEach((account) => {
+      it(`should return true if sort code is ${account.sortCode} and account number is ${account.accountNumber}`, () => {
+        new UkModulusChecking({ accountNumber: account.accountNumber, sortCode: account.sortCode }).isValid().should.be.true();
+      });
+    });
+  });
+});