diff --git a/src/main/java/com/hbm/inventory/gui/GUICalculator.java b/src/main/java/com/hbm/inventory/gui/GUICalculator.java
index 63933eec5..ddfa8e3b7 100644
--- a/src/main/java/com/hbm/inventory/gui/GUICalculator.java
+++ b/src/main/java/com/hbm/inventory/gui/GUICalculator.java
@@ -48,7 +48,7 @@ public class GUICalculator extends GuiScreen {
 		if (!inputField.textboxKeyTyped(p_73869_1_, p_73869_2_))
 			super.keyTyped(p_73869_1_, p_73869_2_);
 
-		String input = inputField.getText().replaceAll("[^\\d+\\-*/^!.()\\sA-Za-z]+", "");
+		String input = inputField.getText().replaceAll("[^\\d+\\-*/%^!.()\\sA-Za-z]+", "");
 
 		if (p_73869_1_ == 13 || p_73869_1_ == 10) { // when pressing enter (CR or LF)
 			if (selectedHist != -1) {
diff --git a/src/main/java/com/hbm/util/Calculator.java b/src/main/java/com/hbm/util/Calculator.java
index 88d91000f..439c7c606 100644
--- a/src/main/java/com/hbm/util/Calculator.java
+++ b/src/main/java/com/hbm/util/Calculator.java
@@ -21,7 +21,7 @@ public class Calculator {
 		for (int i = 0; i < tokens.length; i++) {
 			if (tokens[i] == ' ') continue;
 
-			if (tokens[i] >= '0' && tokens[i] <= '9' || tokens[i] == '.' || (tokens[i] == '-' && (i == 0 || "+-*/^(".contains(String.valueOf(tokens[i - 1]))))) {
+			if (tokens[i] >= '0' && tokens[i] <= '9' || tokens[i] == '.' || (tokens[i] == '-' && (i == 0 || "+-*/%^(".contains(String.valueOf(tokens[i - 1]))))) {
 				StringBuilder buffer = new StringBuilder();
 				if (tokens[i] == '-') {
 					buffer.append('-'); // for negative numbers
@@ -37,7 +37,7 @@ public class Calculator {
 				operators.pop();
 				if (!operators.isEmpty() && operators.peek().length() > 1)
 					values.push(evaluateFunction(operators.pop(), values.pop()));
-			} else if (tokens[i] == '+' || tokens[i] == '-' || tokens[i] == '*' || tokens[i] == '/' || tokens[i] == '^') {
+			} else if (tokens[i] == '+' || tokens[i] == '-' || tokens[i] == '*' || tokens[i] == '/' || tokens[i] == '%' || tokens[i] == '^') {
 				while (!operators.isEmpty() && hasPrecedence(String.valueOf(tokens[i]), operators.peek()))
 					values.push(evaluateOperator(operators.pop().charAt(0), values.pop(), values.pop()));
 				operators.push(Character.toString(tokens[i]));
@@ -67,6 +67,7 @@ public class Calculator {
 			case '-': return y - x;
 			case '*': return y * x;
 			case '/': return y / x;
+			case '%': return y % x;
 
 			case '^': return Math.pow(y, x); // should not happen here, but oh well
 		}
@@ -102,7 +103,7 @@ public class Calculator {
 		char secondChar = second.charAt(0);
 
 		if (secondChar == '(' || secondChar == ')') return false;
-		else return (firstChar != '*' && firstChar != '/' && firstChar != '^') || (secondChar != '+' && secondChar != '-');
+		else return (firstChar != '*' && firstChar != '/' && firstChar != '%' && firstChar != '^') || (secondChar != '+' && secondChar != '-');
 	}
 
 	/** Returns the input with all powers evaluated */