OFFER TO PURCHASE FOR CASH
by
WHITE MOUNTAINS INSURANCE GROUP, LTD.
of
UP TO 500,000 OF ITS COMMON SHARES
at a Purchase Price not greater than $875
or less than $825 per Share

Enclosed for your consideration is the Offer to Purchase dated April 10, 2018, (the "Offer to Purchase") of White Mountains Insurance Group, Ltd., a company organized under the laws of Bermuda (the "Company"), and a related specimen Letter of Transmittal (the "Letter of Transmittal" and, together with the Offer to Purchase, as they may be amended or supplemented from time to time, the "Offer"), pursuant to which the Company is offering to purchase 500,000 of its outstanding common shares, par value $1.00 per share (the "Shares"), or such lesser number of Shares as are properly tendered and not properly withdrawn, at a purchase price not greater than $875 or less than $825 per Share, net to the seller in cash, less any applicable withholding taxes and without interest, upon the terms and subject to the conditions set forth in the Offer. The Company may elect, but shall not be obligated, to purchase additional Shares pursuant to the Offer. The Offer to Purchase and a specimen Letter of Transmittal are being forwarded to you as the beneficial owner of Shares held by us in your account but not registered in your name. A tender of such Shares can be made only by us as the holder of record and only pursuant to your instructions. All capitalized terms used and not defined herein shall have the meanings ascribed to such terms in the Offer to Purchase.

The Company will determine a single per Share price, not greater than $875 or less than $825 per Share, net to the seller in cash, less any applicable withholding taxes and without interest, that it will pay for the Shares properly tendered and not properly withdrawn pursuant to the Offer, taking into account the number of Shares so tendered and the prices specified by tendering shareholders. The Company will select the lowest price not less than $825 (in increments of $1.00) (such price per Share referred to as the "Purchase Price") within the price range specified above that will allow it to purchase 500,000 Shares (or such lesser number as are properly tendered and not properly withdrawn) pursuant to the Offer. The Company will purchase all Shares properly tendered at prices at or below the Purchase Price and not properly withdrawn upon the terms and subject to the conditions set forth in the Offer to Purchase and the related Letter of Transmittal, including the provisions relating to "odd lot" tenders and proration described in the Offer to Purchase. All Shares so purchased by the Company will be purchased at the same price regardless of whether the shareholder tendered at a lower price. In the event that more than 500,000 Shares are tendered in the Offer, the Company may exercise its right to increase the number of Shares sought in the Offer by an amount not exceeding 2% of its outstanding Shares without extending the Expiration Time. The Company also expressly reserves the right, in its sole discretion, to amend the Offer to purchase additional Shares, subject to applicable law. See Sections 1 and 14 of the Offer to Purchase. Shares tendered at prices in excess of the Purchase Price and Shares not purchased because of proration will be returned at the Company's expense to the shareholders who tendered such Shares promptly after the Expiration Time.

If the number of Shares properly tendered and not properly withdrawn before the Expiration Time is less than or equal to 500,000 Shares (or such greater number of Shares as the Company may elect to purchase pursuant to the Offer, subject to applicable law), the Company will, on the terms and subject to the conditions of the Offer, purchase at the purchase price selected by the Company all Shares so tendered.

In the event that more than 500,000 Shares (or such greater number of Shares as the Company may elect to purchase pursuant to the Offer) are properly tendered at or below the Purchase Price and not properly withdrawn, the Company will accept Shares for purchase in the following order of priority, upon the terms and subject to the conditions of the Offer:

Because of the "odd lot" priority and proration tender provisions described above, the Company may not purchase all Shares that you tender even if you tender them at or below the purchase price. See Section 1 of the Offer to Purchase.

WE ARE THE HOLDER OF RECORD OF SHARES HELD FOR YOUR ACCOUNT. A TENDER OF SUCH SHARES CAN BE MADE ONLY BY US AS THE HOLDER OF RECORD AND PURSUANT TO YOUR INSTRUCTIONS. THE LETTER OF TRANSMITTAL IS FURNISHED TO YOU FOR YOUR INFORMATION ONLY AND CANNOT BE USED BY YOU TO TENDER YOUR SHARES HELD BY US FOR YOUR ACCOUNT. WE REQUEST INSTRUCTION AS TO WHETHER YOU WISH US TO TENDER ANY OR ALL OF THE SHARES HELD BY US FOR YOUR ACCOUNT UPON THE TERMS AND SUBJECT TO THE CONDITIONS SET FORTH IN THE OFFER TO PURCHASE AND THE LETTER OF TRANSMITTAL.

If you wish to have us tender any or all of your Shares, please so instruct us by completing, executing, detaching and returning to us the attached Instruction Form. If you authorize us to tender your Shares, we will tender all your Shares unless you specify otherwise on the attached Instruction Form.

The method of delivery of this document is at the election and the risk of the tendering shareholders. If delivery is by mail, registered mail with return receipt requested, properly insured, is recommended. In all other cases, sufficient time should be allowed to ensure timely delivery.

YOUR PROMPT ACTION IS REQUESTED. YOUR INSTRUCTION FORM SHOULD BE FORWARDED TO US IN AMPLE TIME TO PERMIT US TO SUBMIT THE TENDER ON YOUR BEHALF BEFORE THE EXPIRATION OF THE OFFER. PLEASE NOTE THAT THE OFFER AND PRORATION PERIOD AND WITHDRAWAL RIGHTS WILL EXPIRE AT 12:00 MIDNIGHT, NEW YORK CITY TIME, AT THE END OF THE DAY ON MAY 7, 2018, UNLESS THE OFFER IS EXTENDED.

The Offer is being made solely pursuant to the Offer to Purchase and the related Letter of Transmittal and is being made to all holders of the Shares. The Offer is not being made to (nor will tenders be accepted from or on behalf of) holders of Shares residing in any jurisdiction in which the making of the Offer or the acceptance thereof would not be in compliance with the securities or Blue Sky laws of such jurisdiction.

INSTRUCTION FORM WITH RESPECT TO
OFFER TO PURCHASE FOR CASH
by
WHITE MOUNTAINS INSURANCE GROUP, LTD.
of
UP TO 500,000 OF ITS COMMON SHARES
at a Purchase Price not greater than $875
or less than $825 per Share

The undersigned acknowledge(s) receipt of your letter enclosing the Offer to Purchase dated April 10, 2018, and a specimen Letter of Transmittal relating to the Offer by White Mountains Insurance Group, Ltd., a company organized under the laws of Bermuda (the "Company"), to purchase 500,000 of its common shares, par value $1.00 per share ("Shares"), or such lesser number of Shares as are properly tendered and not properly withdrawn, at a purchase price not greater than $875 or less than $825 per Share, net to the seller in cash, less any applicable withholding taxes and without interest.

The undersigned hereby instructs you to tender to the Company the number of Shares indicated below (or, if no number is indicated below, all Shares) that are held by you for the account of the undersigned at the price indicated below, upon the terms and subject to the conditions set forth in the Offer to Purchase and in the related specimen Letter of Transmittal that you have furnished to the undersigned.

In participating in the Offer, the undersigned acknowledges that: (1) the Offer is discretionary and may be extended, modified, suspended or terminated by the Company as provided in the Offer; (2) the undersigned is voluntarily participating in the Offer; (3) the future value of the Company's Shares is unknown and cannot be predicted with certainty; (4) the undersigned has consulted his, her or its tax and financial advisors with regard to how the Offer will impact his, her or its personal situation; (5) any foreign exchange obligations triggered by the undersigned's tender of Shares or the recipient of proceeds are solely his, her or its responsibility; and (6) regardless of any action that the Company takes with respect to any or all income/capital gains tax, social security or other similar tax-related items that are imposed on the undersigned ("Tax Items") related to the Offer and the disposition of Shares, the undersigned acknowledges that the ultimate liability for all Tax Items is and remains his, her or its sole responsibility. In that regard, the undersigned authorizes the Company to withhold, to the extent required by law, all applicable Tax Items legally payable by the undersigned.

The undersigned consents to the collection, use and transfer, in electronic or other form, of the undersigned's personal data as described in this document by and among, as applicable, the Company, its subsidiaries, and third-party administrators for the exclusive purpose of implementing, administering and managing his, her or its participation in the Offer.

The undersigned understands that the Company holds certain personal information about him, her or it, including, as applicable, but not limited to, the undersigned's name, home address and telephone number, date of birth, social security or insurance number or other identification number, nationality, any shares of stock held in the Company, details of all options or any other entitlement to shares outstanding in the undersigned's favor, for the purpose of implementing, administering and managing his, her or its stock ownership ("Data"). The undersigned understands that Data may be transferred to any third parties assisting in the implementation, administration and management of the Offer, that these recipients may be located in his, her or its country or elsewhere, and that the recipient's country may have different data privacy laws and protections than his, her or its country. The undersigned understands that he, she or it may request a list with the names and addresses of any potential recipients of the Data by contacting the Company. The undersigned authorizes the recipients to receive, possess, use, retain and transfer the Data, in electronic or other form, for the purposes of implementing, administering and managing his, her or its participation in the Offer, including any requisite transfer of such Data as may be required to a broker or other third party with whom the undersigned holds any shares of stock. The undersigned understands that Data will be held only as long

as is necessary to implement, administer and manage his, her or its participation in the Offer. The undersigned understands that he, she or it may, at any time, view Data, request additional information about the storage and processing of Data, require any necessary amendments to Data or refuse or withdraw the consents herein, in any case without cost, by contacting in writing the Company. The undersigned understands, however, that refusing or withdrawing his, her or its consent may affect his, her or its ability to participate in the Offer. For more information on the consequences of his, her or its refusal to consent or withdrawal of consent, the undersigned understands that he, she or it may contact the Company.

CHECK ONLY THE BOX UNDER 1 OR 2 BELOW. IF BOTH BOXES UNDER 1 OR 2 ARE CHECKED, OR IF NEITHER OF THE BOXES

UNDER 1 OR 2 ARE CHECKED, THERE IS NO VALID TENDER OF SHARES.
(See Instruction 7 of the Letter of Transmittal)


1. SHARES TENDERED AT A PRICE DETERMINED PURSUANT TO THE OFFER
o		By checking this box INSTEAD ONE OF THE BOXES UNDER 2 BELOW, the undersigned hereby tenders Shares and is willing to accept the purchase price determined by the Company pursuant to the Offer. This action will maximize the chance of having the Company purchase all of the Shares tendered by the undersigned (subject to the possibility of proration). The undersigned understands that this election could result in the tendered shares being purchased at the minimum price of $825 per Share.
—OR—
2. SHARES TENDERED AT A PRICE DETERMINED BY SHAREHOLDER
		By checking ONE of the boxes below INSTEAD OF THE BOX UNDER 1 ABOVE, the undersigned hereby tenders Shares at the price checked the corresponding box in the section below captioned "Price (in Dollars) per Share at which Shares are Being Tendered". This action could result in none of the Shares being purchased if the purchase price for the Shares is less than the price checked. If the purchase price for the Shares is equal to or greater than the price checked, then the Shares purchased by the Company will be purchased at the purchase price. All Shares so purchased by the Company will be purchased at the same price regardless of whether the shareholder tendered at a lower price. A shareholder who wishes to tender Shares at more than one price must complete a separate Letter of Transmittal for each price at which Shares are being tendered. The same Shares cannot be tendered (unless previously withdrawn in accordance with the terms of the Offer) at more than one price.
		PRICE (IN DOLLARS) PER SHARE AT WHICH SHARES ARE BEING TENDERED.
		CHECK ONLY ONE BOX BELOW. IF MORE THAN ONE BOX BELOW IS CHECKED OR IF NO BOX BELOW IS CHECKED, THERE IS NO VALID TENDER OF SHARES.


o	$	825	o	$	826	o	$	827	o	$	828	o	$	829
o	$	830	o	$	831	o	$	832	o	$	833	o	$	834
o	$	835	o	$	836	o	$	837	o	$	838	o	$	839
o	$	840	o	$	841	o	$	842	o	$	843	o	$	844
o	$	845	o	$	846	o	$	847	o	$	848	o	$	849
o	$	850	o	$	851	o	$	852	o	$	853	o	$	854
o	$	855	o	$	856	o	$	857	o	$	858	o	$	859
o	$	860	o	$	861	o	$	862	o	$	863	o	$	864
o	$	865	o	$	866	o	$	867	o	$	868	o	$	869
o	$	870	o	$	871	o	$	872	o	$	873	o	$	874
o	$	875

As described in Section 1 of the Offer to Purchase, under certain conditions, a shareholder owning beneficially or of record an aggregate of fewer than 100 Shares and who tenders all such Shares and satisfies the other requirements set forth in the Offer to Purchase, may have his or her Shares accepted for payment before any proration of other tendered Shares. This preference is not available to partial tenders, or to beneficial or record holders of an aggregate of 100 or more Shares, even if these holders have separate accounts representing fewer than 100 Shares. Accordingly, this section is to be completed only if Shares are being tendered by or on behalf of a person owning, beneficially or of record, an aggregate of fewer than 100 Shares. The undersigned either (check one box):

In addition, the undersigned is tendering all such Shares either (check one box):

SIGN HERE


Name(s) of Record Holder(s):

		(Please Type or Print)


Address(es):


(Include Zip Code)


Area Code and Telephone Number:


Taxpayer ID No(s). or Social Security No(s).:


Date:


Signature(s):


o	$	825	o	$	826	o	$	827	o	$	828	o	$	829
o	$	830	o	$	831	o	$	832	o	$	833	o	$	834
o	$	835	o	$	836	o	$	837	o	$	838	o	$	839
o	$	840	o	$	841	o	$	842	o	$	843	o	$	844
o	$	845	o	$	846	o	$	847	o	$	848	o	$	849
o	$	850	o	$	851	o	$	852	o	$	853	o	$	854
o	$	855	o	$	856	o	$	857	o	$	858	o	$	859
o	$	860	o	$	861	o	$	862	o	$	863	o	$	864
o	$	865	o	$	866	o	$	867	o	$	868	o	$	869
o	$	870	o	$	871	o	$	872	o	$	873	o	$	874
o	$	875


o	$	825	o	$	826	o	$	827	o	$	828	o	$	829
o	$	830	o	$	831	o	$	832	o	$	833	o	$	834
o	$	835	o	$	836	o	$	837	o	$	838	o	$	839
o	$	840	o	$	841	o	$	842	o	$	843	o	$	844
o	$	845	o	$	846	o	$	847	o	$	848	o	$	849
o	$	850	o	$	851	o	$	852	o	$	853	o	$	854
o	$	855	o	$	856	o	$	857	o	$	858	o	$	859
o	$	860	o	$	861	o	$	862	o	$	863	o	$	864
o	$	865	o	$	866	o	$	867	o	$	868	o	$	869
o	$	870	o	$	871	o	$	872	o	$	873	o	$	874
o	$	875


o	$	825	o	$	826	o	$	827	o	$	828	o	$	829
o	$	830	o	$	831	o	$	832	o	$	833	o	$	834
o	$	835	o	$	836	o	$	837	o	$	838	o	$	839
o	$	840	o	$	841	o	$	842	o	$	843	o	$	844
o	$	845	o	$	846	o	$	847	o	$	848	o	$	849
o	$	850	o	$	851	o	$	852	o	$	853	o	$	854
o	$	855	o	$	856	o	$	857	o	$	858	o	$	859
o	$	860	o	$	861	o	$	862	o	$	863	o	$	864
o	$	865	o	$	866	o	$	867	o	$	868	o	$	869
o	$	870	o	$	871	o	$	872	o	$	873	o	$	874
o	$	875