diff --git a/src/lib.rs b/src/lib.rs
index 3ab48492a..99525b9a8 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -179,6 +179,7 @@ pub fn consumer(
     guess_directory: bool,
     binary_path: Option<&Path>,
     ignore_parsing_error: bool,
+    num_threads: usize,
 ) {
     let mut gcov_type = GcovType::Unknown;
 
@@ -290,6 +291,7 @@ pub fn consumer(
                         profraw_paths.as_slice(),
                         binary_path.as_ref().unwrap(),
                         working_dir,
+                        num_threads,
                     ) {
                         Ok(lcovs) => {
                             let mut new_results: Vec<(String, CovResult)> = Vec::new();
diff --git a/src/llvm_tools.rs b/src/llvm_tools.rs
index 40016b4af..af3dcd107 100644
--- a/src/llvm_tools.rs
+++ b/src/llvm_tools.rs
@@ -81,9 +81,11 @@ pub fn llvm_profiles_to_lcov(
     profile_paths: &[PathBuf],
     binary_path: &Path,
     working_dir: &Path,
+    num_threads: usize,
 ) -> Result<Vec<Vec<u8>>, String> {
     let profdata_path = working_dir.join("grcov.profdata");
 
+    let num_threads = num_threads.to_string();
     let args = vec![
         "merge".as_ref(),
         "-f".as_ref(),
@@ -91,6 +93,8 @@ pub fn llvm_profiles_to_lcov(
         "-sparse".as_ref(),
         "-o".as_ref(),
         profdata_path.as_ref(),
+        "--num-threads".as_ref(),
+        num_threads.as_ref(),
     ];
 
     let stdin_paths: String = profile_paths.iter().fold("".into(), |mut a, x| {
@@ -301,6 +305,7 @@ mod tests {
             &[tmp_path.join("default.profraw")],
             &PathBuf::from("src"), // There is no binary file in src
             tmp_path,
+            1,
         );
         assert!(lcovs.is_ok());
         let lcovs = lcovs.unwrap();
@@ -317,6 +322,7 @@ mod tests {
             &[tmp_path.join("default.profraw")],
             &tmp_path.join(binary_path),
             tmp_path,
+            1,
         );
         assert!(lcovs.is_ok(), "Error: {}", lcovs.unwrap_err());
         let lcovs = lcovs.unwrap();
@@ -349,7 +355,8 @@ mod tests {
 
         assert_eq!(status.unwrap().code().unwrap(), 0);
 
-        let lcovs = llvm_profiles_to_lcov(&[profdata_path], &tmp_path.join(binary_path), tmp_path);
+        let lcovs =
+            llvm_profiles_to_lcov(&[profdata_path], &tmp_path.join(binary_path), tmp_path, 1);
 
         assert!(lcovs.is_ok(), "Error: {}", lcovs.unwrap_err());
         let lcovs = lcovs.unwrap();
@@ -396,6 +403,7 @@ mod tests {
             &[path_with, path_without],
             &tmp_path.join(bin_path),
             tmp_path,
+            1,
         );
 
         assert!(lcovs.is_ok(), "Error: {}", lcovs.unwrap_err());
diff --git a/src/main.rs b/src/main.rs
index 1089dd1a3..7a83d0e79 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -465,6 +465,7 @@ fn main() {
                     guess_directory,
                     binary_path.as_deref(),
                     ignore_parsing_error,
+                    num_threads,
                 );
             })
             .unwrap();