diff --git a/lib/snf/src/file.rs b/lib/snf/src/file.rs
index fb04c3e..24e8b73 100644
--- a/lib/snf/src/file.rs
+++ b/lib/snf/src/file.rs
@@ -73,6 +73,16 @@ pub struct Header{
 	#[br(count=block_count+1)]
 	pub block_location:Vec<u64>,
 }
+impl Header{
+	pub const fn calculate_size(block_count:u32)->usize{
+		4 // fourcc
+		+4 // version
+		+8 // priming
+		+16 // resource
+		+4 // block_count
+		+(block_count as usize+1)*8 // block_location
+	}
+}
 
 #[binrw]
 #[brw(little)]
diff --git a/lib/snf/src/map.rs b/lib/snf/src/map.rs
index 69c8b72..d8987c9 100644
--- a/lib/snf/src/map.rs
+++ b/lib/snf/src/map.rs
@@ -412,28 +412,26 @@ pub fn write_map<W:BinWriterExt>(mut writer:W,map:strafesnet_common::map::Comple
 		attributes:map.attributes.into_iter().map(Into::into).collect(),
 		render_configs:map.render_configs.into_iter().map(Into::into).collect(),
 	};
-	let mut file_header=crate::file::Header{
+	//probe header length
+	let mut map_header_data=Vec::new();
+	binrw::BinWrite::write_le(&map_header,&mut std::io::Cursor::new(&mut map_header_data)).map_err(Error::InvalidData)?;
+
+	// calculate final file header
+	let mut offset=crate::file::Header::calculate_size(block_count) as u64;
+	offset+=map_header_data.len() as u64;
+	// priming includes map header
+	let priming=offset;
+	for position in &mut block_location{
+		*position+=offset;
+	}
+	let file_header=crate::file::Header{
 		fourcc:crate::file::FourCC::Map,
 		version:0,
-		priming:0,
+		priming,
 		resource:0,
 		block_count,
 		block_location,
 	};
-	//probe header length
-	let mut file_header_data=Vec::new();
-	binrw::BinWrite::write_le(&file_header,&mut std::io::Cursor::new(&mut file_header_data)).map_err(Error::InvalidData)?;
-	let mut map_header_data=Vec::new();
-	binrw::BinWrite::write_le(&map_header,&mut std::io::Cursor::new(&mut map_header_data)).map_err(Error::InvalidData)?;
-
-	//update file header according to probe data
-	let mut offset=file_header_data.len() as u64;
-	file_header.priming=offset;
-	file_header.block_location[0]=offset;
-	offset+=map_header_data.len() as u64;
-	for position in &mut file_header.block_location[1..]{
-		*position+=offset;
-	}
 
 	//write (updated) file header
 	writer.write_le(&file_header).map_err(Error::InvalidData)?;