diff --git a/OSRef.php b/OSRef.php
index b34ba3379..e4c6d56e0 100644
--- a/OSRef.php
+++ b/OSRef.php
@@ -38,11 +38,6 @@ class OSRef {
      * object from the six-figure grid reference TG514131, the easting would
      * be 651400 and the northing would be 313100.
      *
-     * Grid references with accuracy greater than 1m can be represented
-     * using floating point values for the easting and northing. For example,
-     * a value representing an easting or northing accurate to 1mm would be
-     * given as 651400.0001.
-     *
      * @param int $aEasting the easting of the reference (with 1m accuracy)
      * @param int $aNorthing the northing of the reference (with 1m accuracy)
      */
@@ -67,41 +62,36 @@ public function __toString() {
      * @return string
      */
     public function toSixFigureString() {
-      $hundredkmE = floor($this->easting / 100000);
-      $hundredkmN = floor($this->northing / 100000);
-      $firstLetter = "";
-      if ($hundredkmN < 5) {
-        if ($hundredkmE < 5) {
-          $firstLetter = "S";
-        }
-        else {
-          $firstLetter = "T";
-        }
+
+      $easting = str_pad($this->easting, 6, 0, STR_PAD_LEFT);
+      $northing = str_pad($this->northing, 6, 0, STR_PAD_LEFT);
+
+      $hundredkmE = $easting[0];
+      $hundredkmN = $northing[0];
+
+      if ($hundredkmN < 5 && $hundredkmE < 5) {
+        $firstLetter = 'S';
       }
-      else if ($hundredkmN < 10) {
-        if ($hundredkmE < 5) {
-          $firstLetter = "N";
-        }
-        else {
-          $firstLetter = "O";
-        }
+      else if ($hundredkmN < 5 && $hundredkmE >= 5) {
+        $firstLetter = 'T';
+      }
+      else if ($hundredkmN < 10 && $hundredkmE < 5) {
+        $firstLetter = 'N';
+      }
+      else if ($hundredkmN < 10 && $hundredkmE >= 5) {
+        $firstLetter = 'O';
       }
       else {
-        $firstLetter = "H";
+        $firstLetter = 'H';
       }
 
-      $secondLetter = "";
       $index = 65 + ((4 - ($hundredkmN % 5)) * 5) + ($hundredkmE % 5);
-      $ti = $index;
-      if ($index >= 73) {
+      if ($index >= 73) { //skip the letter I
         $index++;
       }
       $secondLetter = chr($index);
 
-      $e = round(($this->easting - (100000 * $hundredkmE)) / 100);
-      $n = round(($this->northing - (100000 * $hundredkmN)) / 100);
-
-      return sprintf("%s%s%03d%03d", $firstLetter, $secondLetter, $e, $n);
+      return $firstLetter . $secondLetter . substr($easting, 1, 3) . substr($northing, 1, 3);
     }
 
 
diff --git a/README.md b/README.md
index 57ceebb80..140b6d576 100644
--- a/README.md
+++ b/README.md
@@ -28,7 +28,9 @@ Output of calculations have been changed from the original code in the following
  * Eastings and northings are rounded to 1m, and lat/long to 5dp (approx 1m) to avoid any
    misconceptions that precision is the same thing as accuracy
  * When calculating surface distances, a more accurate mean radius is now used rather than
-   that derived from historical definitions of a nautical mile   
+   that derived from historical definitions of a nautical mile 
+ * Corrected calculation of OS 6-figure grid references (rounding instead of truncating meant the
+   grid square was sometimes off by 1) 
 
 Usage
 -----
diff --git a/tests/OSRefTest.php b/tests/OSRefTest.php
index ba65ff1e7..12ccaac03 100644
--- a/tests/OSRefTest.php
+++ b/tests/OSRefTest.php
@@ -38,11 +38,20 @@ public function testLatLngOSWorkedExample() {
     }
     
     public function testToSixFigureString() {
-    
-      $OSRef = new OSRef(530140, 184184);
-   
-      $expected = 'TQ301842';
-    
+
+    $OSRef = new OSRef(530140, 184184);
+
+    $expected = 'TQ301841';
+
+    self::assertEquals($expected, $OSRef->toSixFigureString());
+  }
+
+    public function testToSixFigureString2() {
+
+      $OSRef = new OSRef(439145, 274187);
+
+      $expected = 'SP391741';
+
       self::assertEquals($expected, $OSRef->toSixFigureString());
     }