diff --git a/src/reformat.c b/src/reformat.c
index 0575b4f4b6..47a2600a11 100644
--- a/src/reformat.c
+++ b/src/reformat.c
@@ -430,13 +430,13 @@ avifResult avifImageRGBToYUV(avifImage * image, const avifRGBImage * rgb)
                     int uvI = outerI >> chromaShiftX;
                     int uvJ = outerJ >> chromaShiftY;
                     if (state.yuv.channelBytes > 1) {
-                        uint16_t * pU = (uint16_t *)&uPlane[(uvI * 2) + (uvJ * uRowBytes)];
+                        uint16_t * pU = (uint16_t *)&uPlane[(uvI * 2) + ((size_t)uvJ * uRowBytes)];
                         *pU = (uint16_t)avifYUVColorSpaceInfoUVToUNorm(&state.yuv, avgU);
-                        uint16_t * pV = (uint16_t *)&vPlane[(uvI * 2) + (uvJ * vRowBytes)];
+                        uint16_t * pV = (uint16_t *)&vPlane[(uvI * 2) + ((size_t)uvJ * vRowBytes)];
                         *pV = (uint16_t)avifYUVColorSpaceInfoUVToUNorm(&state.yuv, avgV);
                     } else {
-                        uPlane[uvI + (uvJ * uRowBytes)] = (uint8_t)avifYUVColorSpaceInfoUVToUNorm(&state.yuv, avgU);
-                        vPlane[uvI + (uvJ * vRowBytes)] = (uint8_t)avifYUVColorSpaceInfoUVToUNorm(&state.yuv, avgV);
+                        uPlane[uvI + ((size_t)uvJ * uRowBytes)] = (uint8_t)avifYUVColorSpaceInfoUVToUNorm(&state.yuv, avgU);
+                        vPlane[uvI + ((size_t)uvJ * vRowBytes)] = (uint8_t)avifYUVColorSpaceInfoUVToUNorm(&state.yuv, avgV);
                     }
                 } else if (image->yuvFormat == AVIF_PIXEL_FORMAT_YUV422) {
                     // YUV422, average 2 samples (1x2), twice
@@ -456,13 +456,13 @@ avifResult avifImageRGBToYUV(avifImage * image, const avifRGBImage * rgb)
                         int uvI = outerI >> chromaShiftX;
                         int uvJ = outerJ + bJ;
                         if (state.yuv.channelBytes > 1) {
-                            uint16_t * pU = (uint16_t *)&uPlane[(uvI * 2) + (uvJ * uRowBytes)];
+                            uint16_t * pU = (uint16_t *)&uPlane[(uvI * 2) + ((size_t)uvJ * uRowBytes)];
                             *pU = (uint16_t)avifYUVColorSpaceInfoUVToUNorm(&state.yuv, avgU);
-                            uint16_t * pV = (uint16_t *)&vPlane[(uvI * 2) + (uvJ * vRowBytes)];
+                            uint16_t * pV = (uint16_t *)&vPlane[(uvI * 2) + ((size_t)uvJ * vRowBytes)];
                             *pV = (uint16_t)avifYUVColorSpaceInfoUVToUNorm(&state.yuv, avgV);
                         } else {
-                            uPlane[uvI + (uvJ * uRowBytes)] = (uint8_t)avifYUVColorSpaceInfoUVToUNorm(&state.yuv, avgU);
-                            vPlane[uvI + (uvJ * vRowBytes)] = (uint8_t)avifYUVColorSpaceInfoUVToUNorm(&state.yuv, avgV);
+                            uPlane[uvI + ((size_t)uvJ * uRowBytes)] = (uint8_t)avifYUVColorSpaceInfoUVToUNorm(&state.yuv, avgU);
+                            vPlane[uvI + ((size_t)uvJ * vRowBytes)] = (uint8_t)avifYUVColorSpaceInfoUVToUNorm(&state.yuv, avgV);
                         }
                     }
                 }