Simple Lego brick

// 4x4 Lego-style Brick
// All dimensions in mm based on standard Lego units

// Parameters
width_units = 4;
length_units = 4;

pitch = 8.0;          // Distance between stud centers
brick_height = 9.6;   // Standard brick height
stud_radius = 2.4;    // 4.8mm diameter
stud_height = 1.8;
wall_thickness = 1.5;
internal_tube_outer = 6.51 / 2; // Outer radius of bottom tubes
internal_tube_inner = 4.8 / 2;  // Inner radius to fit studs

$fn = 64; // High resolution for round parts

module brick() {
    total_w = width_units * pitch;
    total_l = length_units * pitch;

    difference() {
        // Main block body
        cube([total_w, total_l, brick_height]);

        // Hollow out the bottom
        translate([wall_thickness, wall_thickness, -1])
            cube([total_w - wall_thickness*2, total_l - wall_thickness*2, brick_height - wall_thickness + 1]);
    }

    // Top Studs
    for (x = [0 : width_units - 1]) {
        for (y = [0 : length_units - 1]) {
            translate([x * pitch + pitch/2, y * pitch + pitch/2, brick_height])
                cylinder(h = stud_height, r = stud_radius);
        }
    }

    // Bottom Tubes (for 4x4 grid, there are 3x3 tubes)
    difference() {
        union() {
            for (x = [1 : width_units - 1]) {
                for (y = [1 : length_units - 1]) {
                    translate([x * pitch, y * pitch, 0])
                        cylinder(h = brick_height - wall_thickness, r = internal_tube_outer);
                }
            }
        }
        // Hollow out the tubes
        for (x = [1 : width_units - 1]) {
            for (y = [1 : length_units - 1]) {
                translate([x * pitch, y * pitch, -0.5])
                    cylinder(h = brick_height - wall_thickness + 1, r = internal_tube_inner);
            }
        }
    }
}

brick();