diff --git a/indivisible.1 b/indivisible.1
index 16648d9..63c1b11 100644
--- a/indivisible.1
+++ b/indivisible.1
@@ -19,6 +19,9 @@ Import prime numbers from \fIFILE\fR
 \fB\-t\fR, \fB\-\-test\fR
 Test if n is a prime instead of generation
 .TP
+\fB\-s\fR, \fB\-\-sieve\fR
+Set a custom sieve size
+.TP
 \fB\-v\fR, \fB\-\-verbose\fR
 Print all found primes
 .TP
diff --git a/src/main.rs b/src/main.rs
index ea05ab1..cca82a2 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -35,12 +35,12 @@ struct Opt {
 	test:bool,
 	#[structopt(help = "Max of the prime to generate or number to test for primality")]
 	num:usize,
+	#[structopt(short, long, default_value = "1000", help = "Set a custom sieve size")]
+	sieve:usize,
 	//#[structopt(short, long, name = "n", default_value = "1", help = "Number of threads to spawn")]
 	//jobs:u64,
 }
 
-const SEGMENT_SIZE:usize = 0x100000000;
-
 fn main() {
 	let opts = Opt::from_args();
 	let mut prime_list = Vec::new();
@@ -69,8 +69,8 @@ fn main() {
 		(*prime_list.last().unwrap() + 1) as usize
 	};
 	while start < opts.num {
-		let end = if start + SEGMENT_SIZE < opts.num {
-			start + SEGMENT_SIZE
+		let end = if start + opts.sieve < opts.num {
+			start + opts.sieve
 		} else {
 			opts.num + 1
 		};
@@ -83,7 +83,7 @@ fn main() {
 		}
 		prime_list.append(&mut new_primes);
 
-		start += SEGMENT_SIZE;
+		start += opts.sieve;
 	}
 
 	if opts.test {