diff --git a/src/pages/tools/number/generic-calc/data/area_volume.ts b/src/pages/tools/number/generic-calc/data/area_volume.ts
index 487b80a..8d71fa1 100644
--- a/src/pages/tools/number/generic-calc/data/area_volume.ts
+++ b/src/pages/tools/number/generic-calc/data/area_volume.ts
@@ -15,6 +15,7 @@ export const areaSphere: GenericCalcType = {
     {
       name: 'r',
       title: 'Radius',
+      formula: 'r = sqrt(A/pi) / 2',
       unit: 'mm',
       default: 1
     }
diff --git a/src/pages/tools/number/generic-calc/data/types.ts b/src/pages/tools/number/generic-calc/data/types.ts
index 9742e5b..45a3d1a 100644
--- a/src/pages/tools/number/generic-calc/data/types.ts
+++ b/src/pages/tools/number/generic-calc/data/types.ts
@@ -36,6 +36,10 @@ export interface GenericCalcType {
     // If present and false,  don't allow user to select this as output
     solvable?: boolean;
 
+    // Alternate rearrangement of the formula, to be used when calculating this.
+    // If missing, the main formula is used with auto derivation.
+    formula?: string;
+
     // Alternates are alternate ways of entering the exact same thing,
     // like the diameter or radius.  The formula for an alternate
     // can use only one variable, always called v, which is the main
diff --git a/src/pages/tools/number/generic-calc/index.tsx b/src/pages/tools/number/generic-calc/index.tsx
index caf99d2..9c51797 100644
--- a/src/pages/tools/number/generic-calc/index.tsx
+++ b/src/pages/tools/number/generic-calc/index.tsx
@@ -25,6 +25,7 @@ import 'nerdamer-prime/Algebra';
 import 'nerdamer-prime/Solve';
 import 'nerdamer-prime/Calculus';
 import Qty from 'js-quantities';
+import { error } from 'console';
 
 function numericSolveEquationFor(
   equation: string,
@@ -444,10 +445,29 @@ export default async function makeTool(
             setResult('Please select a solve for variable');
             return;
           }
-          let expr = nerdamer(calcData.formula);
+          let expr: nerdamer.Expression | null = null;
+
+          for (const i of calcData.variables) {
+            if (i.name === values.outputVariable) {
+              if (i.formula !== undefined) {
+                expr = nerdamer(i.formula);
+              }
+            }
+          }
+
+          if (expr == null) {
+            expr = nerdamer(calcData.formula);
+          }
+          if (expr == null) {
+            throw new Error('No formula found');
+            return;
+          }
 
           Object.keys(values.vars).forEach((key) => {
             if (key === values.outputVariable) return;
+            if (expr === null) {
+              throw new Error('Math fail');
+            }
             expr = expr.sub(key, values.vars[key].value.toString());
           });