diff --git a/src/encrypt.ts b/src/encrypt.ts
index 652f3b2..1be4cc2 100644
--- a/src/encrypt.ts
+++ b/src/encrypt.ts
@@ -165,3 +165,23 @@ export const decryptBase64urlToString = async (
     )
   );
 };
+
+export const getSizeFromOrigToEnc = (x: number) => {
+  if (x < 0 || !Number.isInteger(x)) {
+    throw Error(`x=${x} is not a valid size`);
+  }
+  return (Math.floor(x / 16) + 1) * 16 + 16;
+};
+
+export const getSizeFromEncToOrig = (x: number) => {
+  if (x < 32 || !Number.isInteger(x)) {
+    throw Error(`${x} is not a valid size`);
+  }
+  if (x % 16 !== 0) {
+    throw Error(`${x} is not a valid encrypted file size`);
+  }
+  return {
+    minSize: ((x - 16) / 16 - 1) * 16,
+    maxSize: ((x - 16) / 16 - 1) * 16 + 15,
+  };
+};
diff --git a/tests/encrypt.test.ts b/tests/encrypt.test.ts
index b5f1866..8a4dc4f 100644
--- a/tests/encrypt.test.ts
+++ b/tests/encrypt.test.ts
@@ -8,6 +8,8 @@ import {
   encryptArrayBuffer,
   encryptStringToBase32,
   encryptStringToBase64url,
+  getSizeFromEncToOrig,
+  getSizeFromOrigToEnc,
 } from "../src/encrypt";
 import { base64ToBase64url, bufferToArrayBuffer } from "../src/misc";
 
@@ -110,4 +112,34 @@ describe("Encryption tests", () => {
 
     expect(Buffer.from(dec).equals(Buffer.from(opensslArrBuf))).to.be.true;
   });
+
+  it("should get size from origin to encrypted correctly", () => {
+    expect(() => getSizeFromOrigToEnc(-1)).to.throw();
+    expect(() => getSizeFromOrigToEnc(0.5)).to.throw();
+    expect(getSizeFromOrigToEnc(0)).equals(32);
+    expect(getSizeFromOrigToEnc(15)).equals(32);
+    expect(getSizeFromOrigToEnc(16)).equals(48);
+    expect(getSizeFromOrigToEnc(31)).equals(48);
+    expect(getSizeFromOrigToEnc(32)).equals(64);
+    expect(getSizeFromOrigToEnc(14787203)).equals(14787232);
+  });
+
+  it("should get size from encrypted to origin correctly", () => {
+    expect(() => getSizeFromEncToOrig(-1)).to.throw();
+    expect(() => getSizeFromEncToOrig(30)).to.throw();
+
+    expect(getSizeFromEncToOrig(32)).to.deep.equal({
+      minSize: 0,
+      maxSize: 15,
+    });
+    expect(getSizeFromEncToOrig(48)).to.deep.equal({
+      minSize: 16,
+      maxSize: 31,
+    });
+
+    expect(() => getSizeFromEncToOrig(14787231)).to.throw();
+
+    let { minSize, maxSize } = getSizeFromEncToOrig(14787232);
+    expect(minSize <= 14787203 && 14787203 <= maxSize).to.be.true;
+  });
 });