diff --git a/packages/middleware-flexible-checksums/src/flexibleChecksumsMiddleware.spec.ts b/packages/middleware-flexible-checksums/src/flexibleChecksumsMiddleware.spec.ts
index 492d8c4d3295..f653281759d8 100644
--- a/packages/middleware-flexible-checksums/src/flexibleChecksumsMiddleware.spec.ts
+++ b/packages/middleware-flexible-checksums/src/flexibleChecksumsMiddleware.spec.ts
@@ -4,23 +4,23 @@ import { BuildHandlerArguments } from "@aws-sdk/types";
 import { PreviouslyResolved } from "./configuration";
 import { ChecksumAlgorithm } from "./constants";
 import { flexibleChecksumsMiddleware } from "./flexibleChecksumsMiddleware";
-import { getChecksum } from "./getChecksum";
 import { getChecksumAlgorithmForRequest } from "./getChecksumAlgorithmForRequest";
 import { getChecksumLocationName } from "./getChecksumLocationName";
 import { FlexibleChecksumsMiddlewareConfig } from "./getFlexibleChecksumsPlugin";
 import { hasHeader } from "./hasHeader";
 import { isStreaming } from "./isStreaming";
 import { selectChecksumAlgorithmFunction } from "./selectChecksumAlgorithmFunction";
+import { stringHasher } from "./stringHasher";
 import { validateChecksumFromResponse } from "./validateChecksumFromResponse";
 
 jest.mock("@aws-sdk/protocol-http");
-jest.mock("./getChecksum");
 jest.mock("./getChecksumAlgorithmForRequest");
 jest.mock("./getChecksumLocationName");
 jest.mock("./hasHeader");
 jest.mock("./isStreaming");
-jest.mock("./validateChecksumFromResponse");
 jest.mock("./selectChecksumAlgorithmFunction");
+jest.mock("./stringHasher");
+jest.mock("./validateChecksumFromResponse");
 
 describe(flexibleChecksumsMiddleware.name, () => {
   const mockNext = jest.fn();
@@ -43,10 +43,9 @@ describe(flexibleChecksumsMiddleware.name, () => {
     mockNext.mockResolvedValueOnce(mockResult);
     const { isInstance } = HttpRequest;
     (isInstance as unknown as jest.Mock).mockReturnValue(true);
-    (getChecksum as jest.Mock).mockReturnValue(mockChecksum);
     (getChecksumAlgorithmForRequest as jest.Mock).mockReturnValue(ChecksumAlgorithm.MD5);
     (getChecksumLocationName as jest.Mock).mockReturnValue(mockChecksumLocationName);
-    (hasHeader as jest.Mock).mockReturnValue(false);
+    (hasHeader as jest.Mock).mockReturnValue(true);
     (isStreaming as jest.Mock).mockReturnValue(false);
     (selectChecksumAlgorithmFunction as jest.Mock).mockReturnValue(mockChecksumAlgorithmFunction);
   });
@@ -68,7 +67,6 @@ describe(flexibleChecksumsMiddleware.name, () => {
     describe("request checksum", () => {
       afterEach(() => {
         expect(getChecksumAlgorithmForRequest).toHaveBeenCalledTimes(1);
-        expect(getChecksum).not.toHaveBeenCalled();
       });
 
       it("if checksumAlgorithm is not defined", async () => {
@@ -87,7 +85,6 @@ describe(flexibleChecksumsMiddleware.name, () => {
           ...mockArgs,
           request: { ...mockRequest, headers: mockHeadersWithChecksumHeader },
         };
-        (hasHeader as jest.Mock).mockReturnValue(true);
         await handler(mockArgsWithChecksumHeader);
         expect(getChecksumLocationName).toHaveBeenCalledTimes(1);
         expect(selectChecksumAlgorithmFunction).toHaveBeenCalledTimes(1);
@@ -154,7 +151,15 @@ describe(flexibleChecksumsMiddleware.name, () => {
     });
 
     it("for non-streaming body", async () => {
-      const handler = flexibleChecksumsMiddleware(mockConfig, mockMiddlewareConfig)(mockNext, {});
+      const mockRawChecksum = Buffer.from(mockChecksum);
+      const mockBase64Encoder = jest.fn().mockReturnValue(mockChecksum);
+      (stringHasher as jest.Mock).mockResolvedValue(mockRawChecksum);
+      (hasHeader as jest.Mock).mockReturnValue(false);
+
+      const handler = flexibleChecksumsMiddleware(
+        { ...mockConfig, base64Encoder: mockBase64Encoder },
+        mockMiddlewareConfig
+      )(mockNext, {});
       await handler(mockArgs);
       expect(hasHeader).toHaveBeenCalledTimes(1);
       expect(mockNext).toHaveBeenCalledWith({
@@ -165,7 +170,8 @@ describe(flexibleChecksumsMiddleware.name, () => {
         },
       });
       expect(hasHeader).toHaveBeenCalledWith(mockChecksumLocationName, mockHeaders);
-      expect(getChecksum).toHaveBeenCalledTimes(1);
+      expect(stringHasher).toHaveBeenCalledWith(mockChecksumAlgorithmFunction, mockRequest.body);
+      expect(mockBase64Encoder).toHaveBeenCalledWith(mockRawChecksum);
     });
   });
 
diff --git a/packages/middleware-flexible-checksums/src/flexibleChecksumsMiddleware.ts b/packages/middleware-flexible-checksums/src/flexibleChecksumsMiddleware.ts
index 7f8883bf67f3..c358bc70aca4 100644
--- a/packages/middleware-flexible-checksums/src/flexibleChecksumsMiddleware.ts
+++ b/packages/middleware-flexible-checksums/src/flexibleChecksumsMiddleware.ts
@@ -8,13 +8,13 @@ import {
 } from "@aws-sdk/types";
 
 import { PreviouslyResolved } from "./configuration";
-import { getChecksum } from "./getChecksum";
 import { getChecksumAlgorithmForRequest } from "./getChecksumAlgorithmForRequest";
 import { getChecksumLocationName } from "./getChecksumLocationName";
 import { FlexibleChecksumsMiddlewareConfig } from "./getFlexibleChecksumsPlugin";
 import { hasHeader } from "./hasHeader";
 import { isStreaming } from "./isStreaming";
 import { selectChecksumAlgorithmFunction } from "./selectChecksumAlgorithmFunction";
+import { stringHasher } from "./stringHasher";
 import { validateChecksumFromResponse } from "./validateChecksumFromResponse";
 
 export const flexibleChecksumsMiddleware =
@@ -59,10 +59,10 @@ export const flexibleChecksumsMiddleware =
         };
         delete updatedHeaders["content-length"];
       } else if (!hasHeader(checksumLocationName, headers)) {
-        const checksum = await getChecksum(requestBody, { streamHasher, checksumAlgorithmFn, base64Encoder });
+        const rawChecksum = await stringHasher(checksumAlgorithmFn, requestBody);
         updatedHeaders = {
           ...headers,
-          [checksumLocationName]: checksum,
+          [checksumLocationName]: base64Encoder(rawChecksum),
         };
       }
     }