diff --git a/Cargo.lock b/Cargo.lock
index afbd617..9e3f053 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2,6 +2,24 @@
 # It is not intended for manual editing.
 version = 3
 
+[[package]]
+name = "either"
+version = "1.13.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "60b1af1c220855b6ceac025d3f6ecdd2b7c4894bfe9cd9bda4fbb4bc7c0d4cf0"
+
 [[package]]
 name = "hcn"
-version = "1.0.0"
+version = "1.0.0-multithread"
+dependencies = [
+ "itertools",
+]
+
+[[package]]
+name = "itertools"
+version = "0.13.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "413ee7dfc52ee1a4949ceeb7dbc8a33f2d6c088194d9f922fb8318faf1f01186"
+dependencies = [
+ "either",
+]
diff --git a/Cargo.toml b/Cargo.toml
index 99d8300..59907bf 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,9 +1,10 @@
 [package]
 name = "hcn"
-version = "1.0.0"
+version = "1.0.0-multithread"
 edition = "2021"
 
 [dependencies]
+itertools = "0.13"
 
 [lints.rust]
 unsafe_code = "forbid"
diff --git a/src/main.rs b/src/main.rs
index 7a97081..e37d2e1 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,7 +1,13 @@
 mod sieve;
 
-use std::time::Instant;
+use std::{
+    collections::HashMap,
+    env,
+    thread::{self, available_parallelism},
+    time::Instant,
+};
 
+use itertools::Itertools as _;
 use sieve::get_primes;
 
 const MAX_SIEVED_PRIMES: usize = 100_000_000;
@@ -44,20 +50,71 @@ fn prime_factors(nr: u64, primes: &[u64]) -> u64 {
     num_teilers
 }
 
+fn the_thread(mut start: u64, end: u64, primes: &[u64]) -> HashMap<u64, u64> {
+    let mut max_teilers = 0;
+    let mut max_teilers_hashmap = HashMap::new();
+
+    let mut step_size = 10;
+
+    if start == 0 {
+        start = 2;
+        step_size = 2;
+    }
+
+    for nr in (start..=end).step_by(step_size) {
+        let teilers = prime_factors(nr, primes);
+        if teilers > max_teilers {
+            max_teilers = teilers;
+            max_teilers_hashmap.insert(nr, teilers);
+        }
+    }
+
+    max_teilers_hashmap
+}
+
+fn calculate_chunk_bounds(i: usize, num_threads: usize, max_nr: usize) -> (usize, usize) {
+    let chunk_size = max_nr / num_threads;
+    let start = i * chunk_size;
+    let end = ((i + 1) * chunk_size).min(max_nr);
+    println!("Thread {i}: {start} - {end}");
+    (start, end)
+}
+
 fn main() {
+    let max_nr = env::args()
+        .nth(1)
+        .expect("Usage: hcn <max_nr>")
+        .parse::<usize>()
+        .expect("Invalid max_nr");
+
     let start = Instant::now();
     println!("Precalculating primes...");
     let primes = get_primes(MAX_SIEVED_PRIMES);
     println!("{} primes. Took {:?}", primes.len(), start.elapsed());
 
+    #[allow(clippy::unwrap_used)]
+    let num_threads = available_parallelism().unwrap().get();
+    let mut threads = Vec::with_capacity(num_threads);
+
+    for i in 0..num_threads {
+        let (start, end) = calculate_chunk_bounds(i, num_threads, max_nr);
+        let primes = primes.clone();
+        threads.push(thread::spawn(move || {
+            the_thread(start as u64, end as u64, &primes)
+        }));
+    }
+
+    let mut results = HashMap::new();
+    for thread in threads {
+        let thread_result = thread.join().expect("Thread failed to join (panicked?)");
+        results.extend(thread_result);
+    }
+
     let mut max_teilers = 0;
 
-    let start = Instant::now();
-
-    for nr in (2..1_000_000_000).step_by(2) {
-        let teilers = prime_factors(nr, &primes);
+    for (nr, teilers) in results.into_iter().sorted() {
         if teilers > max_teilers {
-            println!("{nr}: {teilers} ({:?} since start)", start.elapsed());
+            println!("{nr}: {teilers}");
             max_teilers = teilers;
         }
     }