diff --git a/src/nvim/api/buffer.c b/src/nvim/api/buffer.c
index 7f195de959..035e36a2dd 100644
--- a/src/nvim/api/buffer.c
+++ b/src/nvim/api/buffer.c
@@ -529,18 +529,18 @@ void nvim_buf_set_text(uint64_t channel_id, Buffer buffer, Integer start_row, In
 
   // Another call to ml_get_buf() may free the lines, so we make copies
   char *str_at_start = ml_get_buf(buf, (linenr_T)start_row);
-  size_t len_at_start = strlen(str_at_start);
-  str_at_start = arena_memdupz(arena, str_at_start, len_at_start);
-  start_col = start_col < 0 ? (int64_t)len_at_start + start_col + 1 : start_col;
-  VALIDATE_RANGE((start_col >= 0 && (size_t)start_col <= len_at_start), "start_col", {
+  colnr_T len_at_start = ml_get_buf_len(buf, (linenr_T)start_row);
+  str_at_start = arena_memdupz(arena, str_at_start, (size_t)len_at_start);
+  start_col = start_col < 0 ? len_at_start + start_col + 1 : start_col;
+  VALIDATE_RANGE((start_col >= 0 && start_col <= len_at_start), "start_col", {
     return;
   });
 
   char *str_at_end = ml_get_buf(buf, (linenr_T)end_row);
-  size_t len_at_end = strlen(str_at_end);
-  str_at_end = arena_memdupz(arena, str_at_end, len_at_end);
-  end_col = end_col < 0 ? (int64_t)len_at_end + end_col + 1 : end_col;
-  VALIDATE_RANGE((end_col >= 0 && (size_t)end_col <= len_at_end), "end_col", {
+  colnr_T len_at_end = ml_get_buf_len(buf, (linenr_T)end_row);
+  str_at_end = arena_memdupz(arena, str_at_end, (size_t)len_at_end);
+  end_col = end_col < 0 ? len_at_end + end_col + 1 : end_col;
+  VALIDATE_RANGE((end_col >= 0 && end_col <= len_at_end), "end_col", {
     return;
   });
 
@@ -563,12 +563,10 @@ void nvim_buf_set_text(uint64_t channel_id, Buffer buffer, Integer start_row, In
   if (start_row == end_row) {
     old_byte = (bcount_t)end_col - start_col;
   } else {
-    old_byte += (bcount_t)len_at_start - start_col;
+    old_byte += len_at_start - start_col;
     for (int64_t i = 1; i < end_row - start_row; i++) {
       int64_t lnum = start_row + i;
-
-      const char *bufline = ml_get_buf(buf, (linenr_T)lnum);
-      old_byte += (bcount_t)(strlen(bufline)) + 1;
+      old_byte += ml_get_buf_len(buf, (linenr_T)lnum) + 1;
     }
     old_byte += (bcount_t)end_col + 1;
   }
@@ -577,7 +575,7 @@ void nvim_buf_set_text(uint64_t channel_id, Buffer buffer, Integer start_row, In
   String last_item = replacement.items[replacement.size - 1].data.string;
 
   size_t firstlen = (size_t)start_col + first_item.size;
-  size_t last_part_len = len_at_end - (size_t)end_col;
+  size_t last_part_len = (size_t)len_at_end - (size_t)end_col;
   if (replacement.size == 1) {
     firstlen += last_part_len;
   }
@@ -1324,7 +1322,7 @@ static void fix_cursor_cols(win_T *win, linenr_T start_row, colnr_T start_col, l
       // it already (in case virtualedit is active)
       // column might be additionally adjusted below
       // to keep it inside col range if needed
-      colnr_T len = (colnr_T)strlen(ml_get_buf(win->w_buffer, new_end_row));
+      colnr_T len = ml_get_buf_len(win->w_buffer, new_end_row);
       if (win->w_cursor.col < len) {
         win->w_cursor.col = len;
       }
@@ -1424,6 +1422,7 @@ void buf_collect_lines(buf_T *buf, size_t n, linenr_T start, int start_idx, bool
   for (size_t i = 0; i < n; i++) {
     linenr_T lnum = start + (linenr_T)i;
     char *bufstr = ml_get_buf(buf, lnum);
-    push_linestr(lstate, l, bufstr, strlen(bufstr), start_idx + (int)i, replace_nl, arena);
+    size_t bufstrlen = (size_t)ml_get_buf_len(buf, lnum);
+    push_linestr(lstate, l, bufstr, bufstrlen, start_idx + (int)i, replace_nl, arena);
   }
 }
diff --git a/src/nvim/api/extmark.c b/src/nvim/api/extmark.c
index 1b03a97edb..a21cf5b337 100644
--- a/src/nvim/api/extmark.c
+++ b/src/nvim/api/extmark.c
@@ -682,7 +682,7 @@ Integer nvim_buf_set_extmark(Buffer buffer, Integer ns_id, Integer line, Integer
     goto error;
   });
 
-  size_t len = 0;
+  colnr_T len = 0;
 
   if (HAS_KEY(opts, set_extmark, spell)) {
     hl.flags |= (opts->spell) ? kSHSpellOn : kSHSpellOff;
@@ -712,16 +712,16 @@ Integer nvim_buf_set_extmark(Buffer buffer, Integer ns_id, Integer line, Integer
     });
     line = buf->b_ml.ml_line_count;
   } else if (line < buf->b_ml.ml_line_count) {
-    len = opts->ephemeral ? MAXCOL : strlen(ml_get_buf(buf, (linenr_T)line + 1));
+    len = opts->ephemeral ? MAXCOL : ml_get_buf_len(buf, (linenr_T)line + 1);
   }
 
   if (col == -1) {
-    col = (Integer)len;
-  } else if (col > (Integer)len) {
+    col = len;
+  } else if (col > len) {
     VALIDATE_RANGE(!strict, "col", {
       goto error;
     });
-    col = (Integer)len;
+    col = len;
   } else if (col < -1) {
     VALIDATE_RANGE(false, "col", {
       goto error;
@@ -730,7 +730,7 @@ Integer nvim_buf_set_extmark(Buffer buffer, Integer ns_id, Integer line, Integer
 
   if (col2 >= 0) {
     if (line2 >= 0 && line2 < buf->b_ml.ml_line_count) {
-      len = opts->ephemeral ? MAXCOL : strlen(ml_get_buf(buf, (linenr_T)line2 + 1));
+      len = opts->ephemeral ? MAXCOL : ml_get_buf_len(buf, (linenr_T)line2 + 1);
     } else if (line2 == buf->b_ml.ml_line_count) {
       // We are trying to add an extmark past final newline
       len = 0;
@@ -738,11 +738,11 @@ Integer nvim_buf_set_extmark(Buffer buffer, Integer ns_id, Integer line, Integer
       // reuse len from before
       line2 = (int)line;
     }
-    if (col2 > (Integer)len) {
+    if (col2 > len) {
       VALIDATE_RANGE(!strict, "end_col", {
         goto error;
       });
-      col2 = (int)len;
+      col2 = len;
     }
   } else if (line2 >= 0) {
     col2 = 0;
diff --git a/src/nvim/api/private/helpers.c b/src/nvim/api/private/helpers.c
index 1cd98aa0c4..a17e78cc31 100644
--- a/src/nvim/api/private/helpers.c
+++ b/src/nvim/api/private/helpers.c
@@ -524,10 +524,10 @@ String buf_get_text(buf_T *buf, int64_t lnum, int64_t start_col, int64_t end_col
   }
 
   char *bufstr = ml_get_buf(buf, (linenr_T)lnum);
-  size_t line_length = strlen(bufstr);
+  colnr_T line_length = ml_get_buf_len(buf, (linenr_T)lnum);
 
-  start_col = start_col < 0 ? (int64_t)line_length + start_col + 1 : start_col;
-  end_col = end_col < 0 ? (int64_t)line_length + end_col + 1 : end_col;
+  start_col = start_col < 0 ? line_length + start_col + 1 : start_col;
+  end_col = end_col < 0 ? line_length + end_col + 1 : end_col;
 
   if (start_col >= MAXCOL || end_col >= MAXCOL) {
     api_set_error(err, kErrorTypeValidation, "Column index is too high");
@@ -539,7 +539,7 @@ String buf_get_text(buf_T *buf, int64_t lnum, int64_t start_col, int64_t end_col
     return rv;
   }
 
-  if ((size_t)start_col >= line_length) {
+  if (start_col >= line_length) {
     return rv;
   }
 
diff --git a/src/nvim/lua/executor.c b/src/nvim/lua/executor.c
index 1a9bd026b5..08677b77b0 100644
--- a/src/nvim/lua/executor.c
+++ b/src/nvim/lua/executor.c
@@ -1767,7 +1767,7 @@ void ex_luado(exarg_T *const eap)
     lua_pushvalue(lstate, -1);
     const char *const old_line = ml_get_buf(curbuf, l);
     // Get length of old_line here as calling Lua code may free it.
-    const size_t old_line_len = strlen(old_line);
+    const colnr_T old_line_len = ml_get_buf_len(curbuf, l);
     lua_pushstring(lstate, old_line);
     lua_pushnumber(lstate, (lua_Number)l);
     if (nlua_pcall(lstate, 2, 1)) {
@@ -1791,7 +1791,7 @@ void ex_luado(exarg_T *const eap)
         }
       }
       ml_replace(l, new_line_transformed, false);
-      inserted_bytes(l, 0, (int)old_line_len, (int)new_line_len);
+      inserted_bytes(l, 0, old_line_len, (int)new_line_len);
     }
     lua_pop(lstate, 1);
   }
diff --git a/src/nvim/lua/stdlib.c b/src/nvim/lua/stdlib.c
index 8f58fd1a1a..a5262efcfa 100644
--- a/src/nvim/lua/stdlib.c
+++ b/src/nvim/lua/stdlib.c
@@ -107,15 +107,15 @@ static int regex_match_line(lua_State *lstate)
   }
 
   char *line = ml_get_buf(buf, rownr + 1);
-  size_t len = strlen(line);
+  colnr_T len = ml_get_buf_len(buf, rownr + 1);
 
-  if (start < 0 || (size_t)start > len) {
+  if (start < 0 || start > len) {
     return luaL_error(lstate, "invalid start");
   }
 
   char save = NUL;
   if (end >= 0) {
-    if ((size_t)end > len || end < start) {
+    if (end > len || end < start) {
       return luaL_error(lstate, "invalid end");
     }
     save = line[end];
diff --git a/src/nvim/lua/treesitter.c b/src/nvim/lua/treesitter.c
index 25a753b179..6d6ef6c7b9 100644
--- a/src/nvim/lua/treesitter.c
+++ b/src/nvim/lua/treesitter.c
@@ -371,7 +371,7 @@ static const char *input_cb(void *payload, uint32_t byte_index, TSPoint position
     return "";
   }
   char *line = ml_get_buf(bp, (linenr_T)position.row + 1);
-  size_t len = strlen(line);
+  size_t len = (size_t)ml_get_buf_len(bp, (linenr_T)position.row + 1);
   if (position.column > len) {
     *bytes_read = 0;
     return "";