diff --git a/src/layout/OrganisationLookup/validation.ts b/src/layout/OrganisationLookup/validation.ts
index b04b8cccea..d1d1abbee4 100644
--- a/src/layout/OrganisationLookup/validation.ts
+++ b/src/layout/OrganisationLookup/validation.ts
@@ -34,27 +34,28 @@ const orgNrSchema: JSONSchemaType<Pick<Organisation, 'orgNr'>> = {
 };
 
 export function checkValidOrgnNr(orgNr: string): boolean {
-  if (orgNr.length !== 9) {
+  if (orgNr.length !== 9 || !/^\d{9}$/.test(orgNr)) {
     return false;
   }
-  const [a1, a2, a3, a4, a5, a6, a7, a8, a9] = orgNr.split('').map(Number);
-  const allegedCheckDigit = a9;
+  const orgnr_digits = orgNr.split('').map(Number);
+  const k1 = orgnr_digits.at(-1)!;
 
-  const [w1, w2, w3, w4, w5, w6, w7, w8] = [3, 2, 7, 6, 5, 4, 3, 2];
-  const sum = a1 * w1 + a2 * w2 + a3 * w3 + a4 * w4 + a5 * w5 + a6 * w6 + a7 * w7 + a8 * w8;
+  const weights = [3, 2, 7, 6, 5, 4, 3, 2];
 
-  let calculatedCheckDigit = mod11(sum);
-
-  if (calculatedCheckDigit === 11) {
-    calculatedCheckDigit = 0;
+  let sum = 0;
+  for (let i = 0; i < weights.length; i++) {
+    sum += orgnr_digits[i] * weights[i];
   }
 
-  return calculatedCheckDigit === allegedCheckDigit;
+  let calculated_k1 = modularAdditiveInverse(sum, 11);
+  calculated_k1 = calculated_k1 % 11;
+
+  return calculated_k1 === k1;
 }
 
 export const validateOrgnr = ajv.compile(orgNrSchema);
 
-const mod11 = (value: number): number => 11 - (value % 11);
+const modularAdditiveInverse = (value: number, base: number): number => base - (value % base);
 
 const organisationLookupResponseSchema: JSONSchemaType<OrganisationLookupResponse> = {
   type: 'object',
diff --git a/src/layout/PersonLookup/validation.ts b/src/layout/PersonLookup/validation.ts
index 2d656730bc..fb344b24ec 100644
--- a/src/layout/PersonLookup/validation.ts
+++ b/src/layout/PersonLookup/validation.ts
@@ -37,8 +37,8 @@ export function checkValidSsn(ssn: string): boolean {
   }
 
   const digits = ssn.split('').map(Number);
-  const k1 = digits[9];
-  const k2 = digits[10];
+  const k1 = digits.at(-2)!;
+  const k2 = digits.at(-1)!;
 
   // Calculate first control digit (K1)
   const weights1 = [3, 7, 6, 1, 8, 9, 4, 5, 2];
@@ -47,10 +47,8 @@ export function checkValidSsn(ssn: string): boolean {
     sum1 += digits[i] * weights1[i];
   }
 
-  let calculated_k1 = mod11(sum1);
-  if (calculated_k1 === 11) {
-    calculated_k1 = 0;
-  }
+  let calculated_k1 = modularAdditiveInverse(sum1, 11);
+  calculated_k1 = calculated_k1 % 11;
 
   // Calculate second control digit (K2)
   const weights2 = [5, 4, 3, 2, 7, 6, 5, 4, 3, 2];
@@ -63,16 +61,14 @@ export function checkValidSsn(ssn: string): boolean {
     }
   }
 
-  let calculated_k2 = mod11(sum2);
-  if (calculated_k2 === 11) {
-    calculated_k2 = 0;
-  }
+  let calculated_k2 = modularAdditiveInverse(sum2, 11);
+  calculated_k2 = calculated_k2 % 11;
 
   // Validate controls and return result
   return k1 === calculated_k1 && k2 === calculated_k2;
 }
 
-const mod11 = (value: number): number => 11 - (value % 11);
+const modularAdditiveInverse = (value: number, base: number): number => base - (value % base);
 
 export const validateSsn = ajv.compile(ssnSchema);