diff --git a/core/src/net/sf/openrocket/unit/PercentageOfLengthUnit.java b/core/src/net/sf/openrocket/unit/PercentageOfLengthUnit.java
new file mode 100644
index 000000000..5cafe6520
--- /dev/null
+++ b/core/src/net/sf/openrocket/unit/PercentageOfLengthUnit.java
@@ -0,0 +1,99 @@
+package net.sf.openrocket.unit;
+
+import net.sf.openrocket.rocketcomponent.FlightConfiguration;
+import net.sf.openrocket.rocketcomponent.Rocket;
+import net.sf.openrocket.util.BugException;
+
+public class PercentageOfLengthUnit extends GeneralUnit {
+
+    private final FlightConfiguration configuration;
+    private final Rocket rocket;
+
+    private int rocketModId = -1;
+    private int configurationModId = -1;
+
+    private double referenceLength = -1;
+
+    public PercentageOfLengthUnit(FlightConfiguration configuration) {
+        super(0.01, "%");
+        this.configuration = configuration;
+
+        if (configuration == null) {
+            this.rocket = null;
+        } else {
+            this.rocket = configuration.getRocket();
+        }
+    }
+
+    public PercentageOfLengthUnit(Rocket rocket) {
+        super(0.01, "%");
+        this.configuration = null;
+        this.rocket = rocket;
+    }
+
+    public PercentageOfLengthUnit(double reference) {
+        super(0.01, "%");
+        this.configuration = null;
+        this.rocket = null;
+        this.referenceLength = reference;
+
+        if (reference <= 0) {
+            throw new IllegalArgumentException("Illegal reference = " + reference);
+        }
+    }
+
+    @Override
+    public double fromUnit(double value) {
+        checkLength();
+
+        return value * referenceLength * multiplier;
+    }
+
+    @Override
+    public double toUnit(double value) {
+        checkLength();
+
+        return value / referenceLength / multiplier;
+    }
+
+    private void checkLength() {
+        if (configuration != null && configuration.getModID() != configurationModId) {
+            referenceLength = -1;
+            configurationModId = configuration.getModID();
+        }
+        if (rocket != null && rocket.getModID() != rocketModId) {
+            referenceLength = -1;
+            rocketModId = rocket.getModID();
+        }
+        if (referenceLength < 0) {
+            if (configuration != null) {
+                referenceLength = getReferenceLength(configuration);
+            } else if (rocket != null) {
+                referenceLength = getReferenceLength(rocket);
+            } else {
+                throw new BugException("Both rocket and configuration are null");
+            }
+        }
+    }
+
+    /**
+     * Get the reference length of a rocket configuration.
+     *
+     * @param config	the rocket configuration
+     * @return			the reference length of the rocket
+     */
+    public static double getReferenceLength(FlightConfiguration config) {
+        return config.getLength();
+    }
+
+    /**
+     * Get the reference length of a rocket.
+     *
+     * @param rocket	the rocket
+     * @return			the reference length of the rocket
+     */
+    public static double getReferenceLength(Rocket rocket) {
+        return getReferenceLength(rocket.getSelectedConfiguration());
+    }
+
+}
\ No newline at end of file
diff --git a/core/src/net/sf/openrocket/unit/UnitGroup.java b/core/src/net/sf/openrocket/unit/UnitGroup.java
index 4905329c0..99d6c1a58 100644
--- a/core/src/net/sf/openrocket/unit/UnitGroup.java
+++ b/core/src/net/sf/openrocket/unit/UnitGroup.java
@@ -177,13 +177,13 @@ public class UnitGroup {
 		UNITS_STABILITY.addUnit(new GeneralUnit(1, "m"));
 		UNITS_STABILITY.addUnit(new GeneralUnit(0.0254, "in"));
 		UNITS_STABILITY.addUnit(new CaliberUnit((Rocket) null));
+		UNITS_STABILITY.addUnit(new PercentageOfLengthUnit((Rocket) null));
 		UNITS_STABILITY.setDefaultUnit(4);
 		
 		UNITS_STABILITY_CALIBERS = new UnitGroup();
 		UNITS_STABILITY_CALIBERS.addUnit(new GeneralUnit(1, "cal"));
 		UNITS_STABILITY_CALIBERS.setDefaultUnit(0);
-		
-		
+
 		UNITS_VELOCITY = new UnitGroup();
 		UNITS_VELOCITY.addUnit(new GeneralUnit(1, "m/s"));
 		UNITS_VELOCITY.addUnit(new GeneralUnit(1 / 3.6, "km/h"));
@@ -708,25 +708,24 @@ public class UnitGroup {
 	 */
 	private static class StabilityUnitGroup extends UnitGroup {
 		
-		public StabilityUnitGroup(double ref) {
-			this(new CaliberUnit(ref));
-		}
+		public StabilityUnitGroup(double ref) { this(new CaliberUnit(ref), new PercentageOfLengthUnit(ref)); }
 		
 		public StabilityUnitGroup(Rocket rocket) {
-			this(new CaliberUnit(rocket));
+			this(new CaliberUnit(rocket), new PercentageOfLengthUnit(rocket));
 		}
 		
-		public StabilityUnitGroup(FlightConfiguration config) {
-			this(new CaliberUnit(config));
-		}
+		public StabilityUnitGroup(FlightConfiguration config) { this(new CaliberUnit(config), new PercentageOfLengthUnit(config)); }
 		
-		private StabilityUnitGroup(CaliberUnit caliberUnit) {
+		private StabilityUnitGroup(CaliberUnit caliberUnit, PercentageOfLengthUnit percentageOfLengthUnit) {
 			this.units.addAll(UnitGroup.UNITS_STABILITY.units);
 			this.defaultUnit = UnitGroup.UNITS_STABILITY.defaultUnit;
 			for (int i = 0; i < units.size(); i++) {
 				if (units.get(i) instanceof CaliberUnit) {
 					units.set(i, caliberUnit);
 				}
+				if (units.get(i) instanceof PercentageOfLengthUnit) {
+					units.set(i, percentageOfLengthUnit);
+				}
 			}
 		}