diff --git a/painter.js b/painter.js
index 61b669d..c024c4a 100644
--- a/painter.js
+++ b/painter.js
@@ -235,4 +235,28 @@ function setStringPrototype() {
     }
     return res;
   };
+  String.prototype.toRpx = function (minus) {
+    let reg;
+    if (minus) {
+      reg = /^-?[0-9]+([.]{1}[0-9]+){0,1}(rpx|px)$/g;
+    } else {
+      reg = /^[0-9]+([.]{1}[0-9]+){0,1}(rpx|px)$/g;
+    }
+    const results = reg.exec(this);
+    if (!this || !results) {
+      console.error(`The size: ${this} is illegal`);
+      return 0;
+    }
+    const unit = results[2];
+    const value = parseFloat(this);
+
+    let res = 0;
+    if (unit === 'rpx') {
+      res = Math.round(value);
+    } else if (unit === 'px') {
+      res = Math.round(value / screenK);
+    }
+    return res;
+  }  
+  
 }