diff --git a/lib/std/tar.zig b/lib/std/tar.zig
index 9ef2e1fbb2..439a513161 100644
--- a/lib/std/tar.zig
+++ b/lib/std/tar.zig
@@ -140,11 +140,25 @@ pub const Header = struct {
     }
 
     pub fn mode(header: Header) !u32 {
-        return @intCast(try header.numeric(100, 8));
+        return @intCast(try header.octal(100, 8));
     }
 
     pub fn size(header: Header) !u64 {
-        return header.numeric(124, 12);
+        const start = 124;
+        const len = 12;
+        const raw = header.bytes[start..][0..len];
+        //  If the leading byte is 0xff (255), all the bytes of the field
+        //  (including the leading byte) are concatenated in big-endian order,
+        //  with the result being a negative number expressed in two’s
+        //  complement form.
+        if (raw[0] == 0xff) return error.TarNumericValueNegative;
+        // If the leading byte is 0x80 (128), the non-leading bytes of the
+        // field are concatenated in big-endian order.
+        if (raw[0] == 0x80) {
+            if (raw[1] + raw[2] + raw[3] != 0) return error.TarNumericValueTooBig;
+            return std.mem.readInt(u64, raw[4..12], .big);
+        }
+        return try header.octal(start, len);
     }
 
     pub fn chksum(header: Header) !u64 {
@@ -170,22 +184,6 @@ pub const Header = struct {
         return nullStr(header.bytes[start .. start + len]);
     }
 
-    fn numeric(header: Header, start: usize, len: usize) !u64 {
-        const raw = header.bytes[start..][0..len];
-        //  If the leading byte is 0xff (255), all the bytes of the field
-        //  (including the leading byte) are concatenated in big-endian order,
-        //  with the result being a negative number expressed in two’s
-        //  complement form.
-        if (raw[0] == 0xff) return error.TarNumericValueNegative;
-        // If the leading byte is 0x80 (128), the non-leading bytes of the
-        // field are concatenated in big-endian order.
-        if (raw[0] == 0x80) {
-            if (raw[1] + raw[2] + raw[3] != 0) return error.TarNumericValueTooBig;
-            return std.mem.readInt(u64, raw[4..12], .big);
-        }
-        return try header.octal(start, len);
-    }
-
     fn octal(header: Header, start: usize, len: usize) !u64 {
         const raw = header.bytes[start..][0..len];
         // Zero-filled octal number in ASCII. Each numeric field of width w
@@ -756,3 +754,62 @@ test "tar PaxIterator" {
 test {
     _ = @import("tar/test.zig");
 }
+
+test "tar header parse size" {
+    const cases = [_]struct {
+        in: []const u8,
+        want: u64 = 0,
+        err: ?anyerror = null,
+    }{
+        // Test base-256 (binary) encoded values.
+        .{ .in = "", .want = 0 },
+        .{ .in = "\x80", .want = 0 },
+        .{ .in = "\x80\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01", .want = 1 },
+        .{ .in = "\x80\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x02", .want = 0x0102 },
+        .{ .in = "\x80\x00\x00\x00\x01\x02\x03\x04\x05\x06\x07\x08", .want = 0x0102030405060708 },
+        .{ .in = "\x80\x00\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09", .err = error.TarNumericValueTooBig },
+        .{ .in = "\x80\x00\x00\x00\x07\x76\xa2\x22\xeb\x8a\x72\x61", .want = 537795476381659745 },
+
+        // // Test base-8 (octal) encoded values.
+        .{ .in = "00000000227\x00", .want = 0o227 },
+        .{ .in = "  000000227\x00", .want = 0o227 },
+        .{ .in = "00000000228\x00", .err = error.TarHeader },
+        .{ .in = "11111111111\x00", .want = 0o11111111111 },
+    };
+
+    for (cases) |case| {
+        var bytes = [_]u8{0} ** Header.SIZE;
+        @memcpy(bytes[124 .. 124 + case.in.len], case.in);
+        var header = Header{ .bytes = &bytes };
+        if (case.err) |err| {
+            try std.testing.expectError(err, header.size());
+        } else {
+            try std.testing.expectEqual(case.want, try header.size());
+        }
+    }
+}
+
+test "tar header parse mode" {
+    const cases = [_]struct {
+        in: []const u8,
+        want: u64 = 0,
+        err: ?anyerror = null,
+    }{
+        .{ .in = "0000644\x00", .want = 0o644 },
+        .{ .in = "0000777\x00", .want = 0o777 },
+        .{ .in = "7777777\x00", .want = 0o7777777 },
+        .{ .in = "7777778\x00", .err = error.TarHeader },
+        .{ .in = "77777777", .want = 0o77777777 },
+        .{ .in = "777777777777", .want = 0o77777777 },
+    };
+    for (cases) |case| {
+        var bytes = [_]u8{0} ** Header.SIZE;
+        @memcpy(bytes[100 .. 100 + case.in.len], case.in);
+        var header = Header{ .bytes = &bytes };
+        if (case.err) |err| {
+            try std.testing.expectError(err, header.mode());
+        } else {
+            try std.testing.expectEqual(case.want, try header.mode());
+        }
+    }
+}