Take a look and hopefully it works in your code.

private double toRad(double val) { return val * (Math.PI / 180); } public void LatLongToEastNorth(double latitude, double longitude) { //This will not work unless you have your lats and longs in decimal degrees. latitude = toRad(latitude); longitude = toRad(longitude); double a = 6377563.396, b = 6356256.910; // Airy 1830 major & minor semi-axes //double a = 6378137.0, b = 6356752.314245; WGS84 major & minor semi-axes double F0 = 0.9996012717; // NatGrid scale factor on central meridian double lat0 = toRad(49); double lon0 = toRad(-2); // NatGrid true origin double N0 = -100000, E0 = 400000; // northing & easting of true origin, metres double e2 = 1 - (b * b) / (a * a); // eccentricity squared double n = (a - b) / (a + b), n2 = n * n, n3 = n * n * n; double cosLat = Math.Cos(latitude), sinLat = Math.Sin(latitude); double nu = a * F0 / Math.Sqrt(1 - e2 * sinLat * sinLat) ; // transverse radius of curvature double rho = a * F0 * (1 - e2) / Math.Pow(1 - e2 * sinLat * sinLat, 1.5); // meridional radius of curvature double eta2 = nu / rho - 1; double Ma = (1 + n + (5 / 4) * n2 + (5 / 4) * n3) * (latitude - lat0); double Mb = (3 * n + 3 * n * n + (21/8)*n3) * Math.Sin(latitude - lat0) * Math.Cos(latitude + lat0); double Mc = ((15/8)*n2 + (15/8)*n3) * Math.Sin(2 * (latitude - lat0)) * Math.Cos(2 * (latitude + lat0)); double Md = (35 / 24) * n3 * Math.Sin(3 * (latitude - lat0)) * Math.Cos(3 * (latitude + lat0)); double M = b * F0 * (Ma - Mb + Mc - Md); // meridional arc double cos3lat = cosLat * cosLat * cosLat; double cos5lat = cos3lat * cosLat * cosLat; double tan2lat = Math.Tan(latitude) * Math.Tan(latitude); double tan4lat = tan2lat * tan2lat; double I = M + N0; double II = (nu / 2) * sinLat * cosLat; double III = (nu / 24) * sinLat * cos3lat * (5 - tan2lat + 9 * eta2); double IIIA = (nu / 720) * sinLat * cos5lat * (61 - 58 * tan2lat + tan4lat); double IV = nu * cosLat; double V = (nu / 6) * cos3lat * (nu / rho - tan2lat); double VI = (nu / 120) * cos5lat * (5 - 18 * tan2lat + tan4lat + 14 * eta2 - 58 * tan2lat * eta2); double dLon = longitude - lon0; double dLon2 = dLon * dLon, dLon3 = dLon2 * dLon, dLon4 = dLon3 * dLon, dLon5 = dLon4 * dLon, dLon6 = dLon5 * dLon; double N = I + II * dLon2 + III * dLon4 + IIIA * dLon6; //This is the northing double E = E0 + IV * dLon + V * dLon3 + VI * dLon5; //This is the easting }