diff --git a/src/lib/calculator/Calculator.svelte b/src/lib/calculator/Calculator.svelte
index 39d6680..02b03c2 100644
--- a/src/lib/calculator/Calculator.svelte
+++ b/src/lib/calculator/Calculator.svelte
@@ -24,7 +24,11 @@
 	}
 
 	function refreshPrices(calcValues: CalculatorValues) {
-		console.log(calcValues);
+		if (!validateCalculatorValues(calcValues)) {
+			// Calculator values are invalid, we don't want to refresh the prices.
+			return false;
+		}
+
 		if (
 			calcValues.basePrice !== calcValuesCache.basePrice ||
 			calcValues.vatRate !== calcValuesCache.vatRate
@@ -38,6 +42,38 @@
 		refreshCache(calcValues);
 	}
 
+	function validateCalculatorValues(calcValues: CalculatorValues) {
+		let property: keyof typeof calcValues;
+
+		for (property in calcValues) {
+			if (calcValues[property] === null) {
+				return false;
+			}
+		}
+
+		return true;
+	}
+
+	function handleFocus(event: Event) {
+		const element = event.target;
+
+		if (element instanceof HTMLInputElement) {
+			if (element.value === '0') {
+				element.value = '';
+			}
+		}
+	}
+
+	function handleBlur(event: Event) {
+		const element = event.target;
+		
+		if (element instanceof HTMLInputElement) {
+			if (element.value === '') {
+				element.value = '0';
+			}
+		}
+	}
+
 	$: refreshPrices(calcValues);
 
 	const invalidNumberErrorText = 'Zadejte platné číslo.';
@@ -52,6 +88,8 @@
 		max="100"
 		bind:value={calcValues.vatRate}
 		class:invalid={calcValues.vatRate === null}
+		on:focus={handleFocus}
+		on:blur={handleBlur}
 	/>
 	{#if calcValues.vatRate === null}
 		<p class="invalid">{invalidNumberErrorText}</p>
@@ -63,6 +101,8 @@
 		min="0"
 		bind:value={calcValues.basePrice}
 		class:invalid={calcValues.basePrice === null}
+		on:focus={handleFocus}
+		on:blur={handleBlur}
 	/>
 	{#if calcValues.basePrice === null}
 		<p class="invalid">{invalidNumberErrorText}</p>
@@ -74,6 +114,8 @@
 		min="0 "
 		bind:value={calcValues.finalPrice}
 		class:invalid={calcValues.finalPrice === null}
+		on:focus={handleFocus}
+		on:blur={handleBlur}
 	/>
 	{#if calcValues.finalPrice === null}
 		<p class="invalid">{invalidNumberErrorText}</p>
diff --git a/src/lib/calculator/type/CalculatorValues.ts b/src/lib/calculator/type/CalculatorValues.ts
index 9138346..602b590 100644
--- a/src/lib/calculator/type/CalculatorValues.ts
+++ b/src/lib/calculator/type/CalculatorValues.ts
@@ -1,6 +1,6 @@
 export default interface CalculatorValues {
-	basePrice: number;
-	vatRate: number;
-	vatAmount: number;
-	finalPrice: number;
+	basePrice: number|null;
+	vatRate: number|null;
+	vatAmount: number|null;
+	finalPrice: number|null;
 }