diff --git a/quickfixj-core/src/main/java/quickfix/FieldMap.java b/quickfixj-core/src/main/java/quickfix/FieldMap.java
index 6f5c9ef272..d78bf5cf2f 100644
--- a/quickfixj-core/src/main/java/quickfix/FieldMap.java
+++ b/quickfixj-core/src/main/java/quickfix/FieldMap.java
@@ -31,6 +31,7 @@
 import quickfix.field.converter.UtcDateOnlyConverter;
 import quickfix.field.converter.UtcTimeOnlyConverter;
 import quickfix.field.converter.UtcTimestampConverter;
+import org.quickfixj.CharsetSupport;
 
 import java.io.Serializable;
 import java.math.BigDecimal;
@@ -471,8 +472,9 @@ protected void calculateString(StringBuilder buffer, int[] preFields, int[] post
             } else if (isGroupField(tag) && isOrderedField(tag, fieldOrder)
                     && getGroupCount(tag) > 0) {
                 appendField(buffer, field);
-                for (Group group : getGroups(tag)) {
-                    group.calculateString(buffer, preFields, postFields);
+                List<Group> groups = getGroups(tag);
+                for (int i = 0; i < groups.size(); i++) {
+                    groups.get(i).calculateString(buffer, preFields, postFields);
                 }
             }
         }
@@ -483,10 +485,10 @@ && getGroupCount(tag) > 0) {
                 final List<Group> groups = entry.getValue();
                 int groupCount = groups.size();
                 if (groupCount > 0) {
-                    final IntField countField = new IntField(groupCountTag.intValue(), groupCount);
-                    appendField(buffer, countField);
-                    for (Group group : groups) {
-                        group.calculateString(buffer, preFields, postFields);
+                    buffer.append(NumbersCache.get(groupCountTag)).append('=');
+                    buffer.append(NumbersCache.get(groupCount)).append('\001');
+                    for (int i = 0; i < groups.size(); i++) {
+                        groups.get(i).calculateString(buffer, preFields, postFields);
                     }
                 }
             }
@@ -499,6 +501,8 @@ && getGroupCount(tag) > 0) {
         }
     }
 
+    private static final boolean IS_STRING_EQUIVALENT = CharsetSupport.isStringEquivalent(CharsetSupport.getCharsetInstance());
+
     int calculateLength() {
         int result = 0;
         for (final Field<?> field : fields.values()) {
@@ -512,17 +516,27 @@ int calculateLength() {
         for (Entry<Integer, List<Group>> entry : groups.entrySet()) {
             final List<Group> groupList = entry.getValue();
             if (!groupList.isEmpty()) {
-                final IntField groupField = new IntField(entry.getKey());
-                groupField.setValue(groupList.size());
-                result += groupField.getLength();
-                for (final Group group : groupList) {
-                    result += group.calculateLength();
+                if(IS_STRING_EQUIVALENT) {
+                    result += getStringLength(entry.getKey()) + getStringLength(groupList.size()) + 2;
+                } else {
+                    result += MessageUtils.length(CharsetSupport.getCharsetInstance(), NumbersCache.get(entry.getKey()));
+                    result += MessageUtils.length(CharsetSupport.getCharsetInstance(), NumbersCache.get(groupList.size()));
+                    result += 2;
+                }
+                for (int i = 0; i < groupList.size(); i++) {
+                    result += groupList.get(i).calculateLength();
                 }
             }
         }
         return result;
     }
 
+    private static int getStringLength(int num) {
+        if(num == 0)
+            return 1;
+        return (int)(num > 0 ? Math.log10(num) + 1 : Math.log10(-num) + 2);
+    }
+
     int calculateChecksum() {
         int result = 0;
         for (final Field<?> field : fields.values()) {
@@ -534,11 +548,21 @@ int calculateChecksum() {
         for (Entry<Integer, List<Group>> entry : groups.entrySet()) {
             final List<Group> groupList = entry.getValue();
             if (!groupList.isEmpty()) {
-                final IntField groupField = new IntField(entry.getKey());
-                groupField.setValue(groupList.size());
-                result += groupField.getChecksum();
-                for (final Group group : groupList) {
-                    result += group.calculateChecksum();
+                if(IS_STRING_EQUIVALENT) {
+                    String value = NumbersCache.get(entry.getKey());
+                    for (int i = value.length(); i-- != 0;)
+                        result += value.charAt(i);
+                    value = NumbersCache.get(groupList.size());
+                    for (int i = value.length(); i-- != 0;)
+                        result += value.charAt(i);
+                    result += '=' + 1;
+                } else {
+                    final IntField groupField = new IntField(entry.getKey());
+                    groupField.setValue(groupList.size());
+                    result += groupField.getChecksum();
+                }
+                for (int i = 0; i < groupList.size(); i++) {
+                    result += groupList.get(i).calculateChecksum();
                 }
             }
         }
@@ -662,4 +686,5 @@ public boolean hasGroup(Group group) {
         return hasGroup(group.getFieldTag());
     }
 
+    
 }
diff --git a/quickfixj-core/src/main/java/quickfix/NumbersCache.java b/quickfixj-core/src/main/java/quickfix/NumbersCache.java
index 1a3725884a..fdacaa695b 100644
--- a/quickfixj-core/src/main/java/quickfix/NumbersCache.java
+++ b/quickfixj-core/src/main/java/quickfix/NumbersCache.java
@@ -48,16 +48,18 @@ public static String get(int i) {
         return String.valueOf(i);
     }
 
-    /**
-     * Get the string representing the given double if it's an integer
-     *
-     * @param d the double to convert
-     * @return the String representing the double or null if the double is not an integer
-     */
+
+     /**
+      * Get the string representing the given double if it's an integer
+      *
+      * @param d the double to convert
+      * @return the String representing the double or null if the double is not an integer
+      */
     public static String get(double d) {
         long l = (long)d;
         if (d == (double)l)
             return get(l);
         return null;
-    }
+	  }
+
 }